One list in ConPat (part of #25127)

This patch changes PrefixCon to use one list instead of two:

	-data HsConDetails tyarg arg rec
	-  = PrefixCon [tyarg] [arg]
	+data HsConDetails arg rec
	+  = PrefixCon [arg]
	   | RecCon    rec
	   | InfixCon  arg arg

The [tyarg] list is now gone. To understand the effect of this change,
recall that there are three instantiations of HsConDetails:

1. type HsConPatDetails p =
      HsConDetails (HsConPatTyArg (NoGhcTc p))  -- tyarg
                   (LPat p)                     -- arg
                   (HsRecFields p (LPat p))     -- rec

2. type HsConDeclH98Details pass =
      HsConDetails Void                              -- tyarg
                   (HsScaled pass (LBangType pass))  -- arg
                   (XRec pass [LConDeclField pass])  -- rec

3. type HsPatSynDetails pass =
      HsConDetails Void                     -- tyarg
                   (LIdP pass)              -- arg
                   [RecordPatSynField pass] -- rec

In cases (2) and (3), tyarg was instantiated to Void, so the [tyarg]
list was always empty. Its removal is basically a no-op.

The interesting case is (1), which is used in ConPat to represent
pattern matching of the form (MkE @tp1 @tp2 p1 p2).
With this patch, its representation is changed as follows:

  ConPat "MkE" [tp1, tp2] [p1, p2]               -- old
  ConPat "MkE" [InvisP tp1, InvisP tp2, p1, p2]  -- new

The new mixed-list representation is consintent with lambdas, where
InvisP is already used to deal with \ @tp1 @tp2 p1 p2 -> body.

The immediate effect of the new representation is an improvement to
error messages. Consider the pattern (Con x @t y). Previously it
resulted in a parse error because @t could not occur after x. Now it is
reported as [GHC-14964]. Test case: TyAppPat_MisplacedApplication.

In the long term, this is intended as preparation for #18389 and #25127,
which would make (Con x @t y) potentially valid, e.g. if its type is
  Con :: forall a -> forall b. (a, b) -> T

The TH AST is left unchanged for the moment to avoid breakage.