TTG: Rework and improve splices

This commit redefines the structure of Splices in the AST.

We get rid of `HsSplice` which used to represent typed and untyped
splices, quasi quotes, and the result of splicing either an expression,
a type or a pattern.

Instead we have `HsUntypedSplice` which models an untyped splice or a
quasi quoter, which works in practice just like untyped splices.

The `HsExpr` constructor `HsSpliceE` which used to be constructed with
an `HsSplice` is split into `HsTypedSplice` and `HsUntypedSplice`. The
former is directly constructed with an `HsExpr` and the latter now takes
an `HsUntypedSplice`.

Both `HsType` and `Pat` constructors `HsSpliceTy` and `SplicePat` now
take an `HsUntypedSplice` instead of a `HsSplice` (remember only
/untyped splices/ can be spliced as types or patterns).

The result of splicing an expression, type, or pattern is now
comfortably stored in the extension fields `XSpliceTy`, `XSplicePat`,
`XUntypedSplice` as, respectively, `HsUntypedSpliceResult (HsType
GhcRn)`, `HsUntypedSpliceResult (Pat GhcRn)`, and `HsUntypedSpliceResult
(HsExpr GhcRn)`

Overall the TTG extension points are now better used to
make invalid states unrepresentable and model the progression between
stages better.

See Note [Lifecycle of an untyped splice, and PendingRnSplice]
and Note [Lifecycle of an typed splice, and PendingTcSplice] for more
details.

Updates haddock submodule

Fixes #21263

-------------------------
Metric Decrease:
    hard_hole_fits
-------------------------

1 files changed, 34 insertions, 16 deletions

diff --git a/testsuite/tests/perf/compiler/hard_hole_fits.stderr b/testsuite/tests/perf/compiler/hard_hole_fits.stderr
index 55c267076d..4caef50d4c 100644
--- a/testsuite/tests/perf/compiler/hard_hole_fits.stderr
+++ b/testsuite/tests/perf/compiler/hard_hole_fits.stderr
@@ -135,7 +135,7 @@ hard_hole_fits.hs:20:24: warning: [-Wtyped-holes (in -Wdefault)]
 hard_hole_fits.hs:21:40: warning: [-Wtyped-holes (in -Wdefault)]
     • Found hole: _ :: Int
     • In an equation for ‘testMe’:
-           testMe (HsLamCase xlc lc_variant mg) = _
+          testMe (HsLamCase xlc lc_variant mg) = _
     • Relevant bindings include
         mg :: MatchGroup GhcPs (LHsExpr GhcPs)
           (bound at hard_hole_fits.hs:21:34)
@@ -577,13 +577,13 @@ hard_hole_fits.hs:42:35: warning: [-Wtyped-holes (in -Wdefault)]
           (imported from ‘Prelude’ at hard_hole_fits.hs:8:8-20
            (and originally defined in ‘GHC.Enum’))
 
-hard_hole_fits.hs:43:29: warning: [-Wtyped-holes (in -Wdefault)]
+hard_hole_fits.hs:43:32: warning: [-Wtyped-holes (in -Wdefault)]
     • Found hole: _ :: Int
-    • In an equation for ‘testMe’: testMe (HsSpliceE xse hs) = _
+    • In an equation for ‘testMe’: testMe (HsTypedSplice xs hs) = _
     • Relevant bindings include
-        hs :: HsSplice GhcPs (bound at hard_hole_fits.hs:43:23)
-        xse :: Language.Haskell.Syntax.Extension.XSpliceE GhcPs
-          (bound at hard_hole_fits.hs:43:19)
+        hs :: LHsExpr GhcPs (bound at hard_hole_fits.hs:43:26)
+        xs :: Language.Haskell.Syntax.Extension.XTypedSplice GhcPs
+          (bound at hard_hole_fits.hs:43:23)
         testMe :: HsExpr GhcPs -> Int (bound at hard_hole_fits.hs:14:1)
       Valid hole fits include
         maxBound :: forall a. Bounded a => a
@@ -595,15 +595,33 @@ hard_hole_fits.hs:43:29: warning: [-Wtyped-holes (in -Wdefault)]
           (imported from ‘Prelude’ at hard_hole_fits.hs:8:8-20
            (and originally defined in ‘GHC.Enum’))
 
-hard_hole_fits.hs:44:29: warning: [-Wtyped-holes (in -Wdefault)]
+hard_hole_fits.hs:44:34: warning: [-Wtyped-holes (in -Wdefault)]
+    • Found hole: _ :: Int
+    • In an equation for ‘testMe’: testMe (HsUntypedSplice xs hs) = _
+    • Relevant bindings include
+        hs :: HsUntypedSplice GhcPs (bound at hard_hole_fits.hs:44:28)
+        xs :: Language.Haskell.Syntax.Extension.XUntypedSplice GhcPs
+          (bound at hard_hole_fits.hs:44:25)
+        testMe :: HsExpr GhcPs -> Int (bound at hard_hole_fits.hs:14:1)
+      Valid hole fits include
+        maxBound :: forall a. Bounded a => a
+          with maxBound @Int
+          (imported from ‘Prelude’ at hard_hole_fits.hs:8:8-20
+           (and originally defined in ‘GHC.Enum’))
+        minBound :: forall a. Bounded a => a
+          with minBound @Int
+          (imported from ‘Prelude’ at hard_hole_fits.hs:8:8-20
+           (and originally defined in ‘GHC.Enum’))
+
+hard_hole_fits.hs:45:29: warning: [-Wtyped-holes (in -Wdefault)]
     • Found hole: _ :: Int
     • In an equation for ‘testMe’: testMe (HsProc xp pat gl) = _
     • Relevant bindings include
-        gl :: LHsCmdTop GhcPs (bound at hard_hole_fits.hs:44:23)
+        gl :: LHsCmdTop GhcPs (bound at hard_hole_fits.hs:45:23)
         pat :: Language.Haskell.Syntax.Pat.LPat GhcPs
-          (bound at hard_hole_fits.hs:44:19)
+          (bound at hard_hole_fits.hs:45:19)
         xp :: Language.Haskell.Syntax.Extension.XProc GhcPs
-          (bound at hard_hole_fits.hs:44:16)
+          (bound at hard_hole_fits.hs:45:16)
         testMe :: HsExpr GhcPs -> Int (bound at hard_hole_fits.hs:14:1)
       Valid hole fits include
         maxBound :: forall a. Bounded a => a
@@ -615,13 +633,13 @@ hard_hole_fits.hs:44:29: warning: [-Wtyped-holes (in -Wdefault)]
           (imported from ‘Prelude’ at hard_hole_fits.hs:8:8-20
            (and originally defined in ‘GHC.Enum’))
 
-hard_hole_fits.hs:45:27: warning: [-Wtyped-holes (in -Wdefault)]
+hard_hole_fits.hs:46:27: warning: [-Wtyped-holes (in -Wdefault)]
     • Found hole: _ :: Int
     • In an equation for ‘testMe’: testMe (HsStatic xs gl) = _
     • Relevant bindings include
-        gl :: LHsExpr GhcPs (bound at hard_hole_fits.hs:45:21)
+        gl :: LHsExpr GhcPs (bound at hard_hole_fits.hs:46:21)
         xs :: Language.Haskell.Syntax.Extension.XStatic GhcPs
-          (bound at hard_hole_fits.hs:45:18)
+          (bound at hard_hole_fits.hs:46:18)
         testMe :: HsExpr GhcPs -> Int (bound at hard_hole_fits.hs:14:1)
       Valid hole fits include
         maxBound :: forall a. Bounded a => a
@@ -633,16 +651,16 @@ hard_hole_fits.hs:45:27: warning: [-Wtyped-holes (in -Wdefault)]
           (imported from ‘Prelude’ at hard_hole_fits.hs:8:8-20
            (and originally defined in ‘GHC.Enum’))
 
-hard_hole_fits.hs:46:1: warning: [-Woverlapping-patterns (in -Wdefault)]
+hard_hole_fits.hs:47:1: warning: [-Woverlapping-patterns (in -Wdefault)]
     Pattern match is redundant
     In an equation for ‘testMe’: testMe (XExpr xe) = ...
 
-hard_hole_fits.hs:46:21: warning: [-Wtyped-holes (in -Wdefault)]
+hard_hole_fits.hs:47:21: warning: [-Wtyped-holes (in -Wdefault)]
     • Found hole: _ :: Int
     • In an equation for ‘testMe’: testMe (XExpr xe) = _
     • Relevant bindings include
         xe :: Language.Haskell.Syntax.Extension.XXExpr GhcPs
-          (bound at hard_hole_fits.hs:46:15)
+          (bound at hard_hole_fits.hs:47:15)
         testMe :: HsExpr GhcPs -> Int (bound at hard_hole_fits.hs:14:1)
       Valid hole fits include
         maxBound :: forall a. Bounded a => a