Always use rnImplicitBndrs to bring implicit tyvars into scope

This implements a first step towards #16762 by changing the renamer
to always use `rnImplicitBndrs` to bring implicitly bound type
variables into scope. The main change is in `rnFamInstEqn` and
`bindHsQTyVars`, which previously used _ad hoc_ methods of binding
their implicit tyvars.

There are a number of knock-on consequences:

* One of the reasons that `rnFamInstEqn` used an _ad hoc_ binding
  mechanism was to give more precise source locations in
  `-Wunused-type-patterns` warnings. (See
  https://gitlab.haskell.org/ghc/ghc/issues/16762#note_273343 for an
  example of this.) However, these warnings are actually a little
  _too_ precise, since implicitly bound type variables don't have
  exact binding sites like explicitly bound type variables do.
  A similar problem existed for
  "`Different names for the same type variable`" errors involving
  implicit tyvars bound by `bindHsQTyVars`.
  Therefore, we simply accept the less precise (but more accurate)
  source locations from `rnImplicitBndrs` in `rnFamInstEqn` and
  `bindHsQTyVars`. See
  `Note [Source locations for implicitly bound type variables]` in
  `GHC.Rename.HsType` for the full story.
* In order for `rnImplicitBndrs` to work in `rnFamInstEqn`, it needs
  to be able to look up names from the parent class (in the event
  that we are renaming an associated type family instance). As a
  result, `rnImplicitBndrs` now takes an argument of type
  `Maybe assoc`, which is `Just` in the event that a type family
  instance is associated with a class.
* Previously, GHC kept track of three type synonyms for free type
  variables in the renamer: `FreeKiTyVars`, `FreeKiTyVarsDups`
  (which are allowed to contain duplicates), and
  `FreeKiTyVarsNoDups` (which contain no duplicates). However, making
  is a distinction between `-Dups` and `-NoDups` is now pointless, as
  all code that returns `FreeKiTyVars{,Dups,NoDups}` will eventually
  end up being passed to `rnImplicitBndrs`, which removes duplicates.
  As a result, I decided to just get rid of `FreeKiTyVarsDups` and
  `FreeKiTyVarsNoDups`, leaving only `FreeKiTyVars`.
* The `bindLRdrNames` and `deleteBys` functions are now dead code, so
  I took the liberty of removing them.

4 files changed, 5 insertions, 5 deletions

diff --git a/testsuite/tests/polykinds/T11203.stderr b/testsuite/tests/polykinds/T11203.stderr
index e6f30c0335..decefbd1fa 100644
--- a/testsuite/tests/polykinds/T11203.stderr
+++ b/testsuite/tests/polykinds/T11203.stderr
@@ -1,4 +1,4 @@
 
-T11203.hs:7:24: error:
+T11203.hs:7:9: error:
     • Different names for the same type variable: ‘k1’ and ‘k2’
     • In the data declaration for ‘Q’
diff --git a/testsuite/tests/polykinds/T11821a.stderr b/testsuite/tests/polykinds/T11821a.stderr
index 50d58df8f5..cc18f2c0fd 100644
--- a/testsuite/tests/polykinds/T11821a.stderr
+++ b/testsuite/tests/polykinds/T11821a.stderr
@@ -1,4 +1,4 @@
 
-T11821a.hs:4:31: error:
+T11821a.hs:4:16: error:
     • Different names for the same type variable: ‘k1’ and ‘k2’
     • In the type declaration for ‘SameKind’
diff --git a/testsuite/tests/polykinds/T17841.stderr b/testsuite/tests/polykinds/T17841.stderr
index 11243a4322..de33036dcf 100644
--- a/testsuite/tests/polykinds/T17841.stderr
+++ b/testsuite/tests/polykinds/T17841.stderr
@@ -3,7 +3,7 @@ T17841.hs:7:45: error:
     • Expected a type, but ‘t’ has kind ‘k2’
       ‘k2’ is a rigid type variable bound by
         the class declaration for ‘Foo’
-        at T17841.hs:7:17
+        at T17841.hs:7:12-17
     • In the kind ‘t’
       In the first argument of ‘Proxy’, namely ‘(a :: t)’
       In the type signature: foo :: Proxy (a :: t)
diff --git a/testsuite/tests/polykinds/T7224.stderr b/testsuite/tests/polykinds/T7224.stderr
index c9d2236206..ca1f32c1c2 100644
--- a/testsuite/tests/polykinds/T7224.stderr
+++ b/testsuite/tests/polykinds/T7224.stderr
@@ -3,7 +3,7 @@ T7224.hs:6:19: error:
     • Expected kind ‘i’, but ‘i’ has kind ‘*’
       ‘i’ is a rigid type variable bound by
         the class declaration for ‘PMonad'’
-        at T7224.hs:5:21
+        at T7224.hs:5:16-36
     • In the first argument of ‘m’, namely ‘i’
       In the type signature: ret' :: a -> m i i a
       In the class declaration for ‘PMonad'’
@@ -12,7 +12,7 @@ T7224.hs:7:14: error:
     • Expected kind ‘i’, but ‘i’ has kind ‘*’
       ‘i’ is a rigid type variable bound by
         the class declaration for ‘PMonad'’
-        at T7224.hs:5:21
+        at T7224.hs:5:16-36
     • In the first argument of ‘m’, namely ‘i’
       In the type signature:
         bind' :: m i j a -> (a -> m j k b) -> m i k b