Smarter HsType pretty-print for promoted datacons

Fix Trac #15898, by being smarter about when to print
a space before a promoted data constructor, in a HsType.
I had to implement a mildly tiresome function
    HsType.lhsTypeHasLeadingPromotionQuote
It has multiple cases, of course, but it's very simple.

The patch improves the error-message output in a bunch of
cases, and (to my surprise) actually fixes a bug in the
output of T14343 (Trac #14343), thus

  -  In the expression: _ :: Proxy '('( 'True,  'False),  'False)
  +  In the expression: _ :: Proxy '( '( 'True, 'False), 'False)

I discovered that there were two copies of the PromotionFlag
type (a boolean, with helpfully named data cons), one in
IfaceType and one in HsType.  So I combined into one,
PromotionFlag, and moved it to BasicTypes.  That's why
quite a few files are touched, but it's all routine.

1 files changed, 19 insertions, 0 deletions

diff --git a/compiler/basicTypes/BasicTypes.hs b/compiler/basicTypes/BasicTypes.hs
index cf56957f7b..200e5c9b8a 100644
--- a/compiler/basicTypes/BasicTypes.hs
+++ b/compiler/basicTypes/BasicTypes.hs
@@ -28,6 +28,7 @@ module BasicTypes(
 
         Alignment,
 
+        PromotionFlag(..), isPromoted,
         FunctionOrData(..),
 
         WarningTxt(..), pprWarningTxtForMsg, StringLiteral(..),
@@ -270,6 +271,24 @@ unSwap :: SwapFlag -> (a->a->b) -> a -> a -> b
 unSwap NotSwapped f a b = f a b
 unSwap IsSwapped  f a b = f b a
 
+
+{- *********************************************************************
+*                                                                      *
+           Promotion flag
+*                                                                      *
+********************************************************************* -}
+
+-- | Is a TyCon a promoted data constructor or just a normal type constructor?
+data PromotionFlag
+  = NotPromoted
+  | IsPromoted
+  deriving ( Eq, Data )
+
+isPromoted :: PromotionFlag -> Bool
+isPromoted IsPromoted  = True
+isPromoted NotPromoted = False
+
+
 {-
 ************************************************************************
 *                                                                      *