Wildcard binders in type declarations (#23501)

Add support for wildcard binders in type declarations:

type Const a b = a   -- BEFORE: the `b` had to be named
                     --         even if unused on the RHS

type Const a _ = a   -- AFTER: the compiler accepts
                     --        a wildcard binder `_`

The new feature is part of GHC Proposal #425 "Invisible binders in type declarations", and more specifically its amendment #641.

Just like a named binder, a wildcard binder _ may be:

• plain: _
• kinded: (_ :: k -> Type)
• invisible, plain: @_
• invisible, kinded: @(_ :: k -> Type)

Those new forms of binders are allowed to occur on the LHSs of data, newtype, type, class, and type/data family declarations:

data D _ = ...
newtype N _ = ...
type T _ = ...
class C _ where ...
type family F _
data family DF _

(Test case: testsuite/tests/typecheck/should_compile/T23501a.hs)

However, we choose to reject them in forall telescopes and type family result variable binders (the latter being part of the TypeFamilyDependencies extension):

type family Fd a = _    -- disallowed  (WildcardBndrInTyFamResultVar)
fn :: forall _. Int     -- disallowed  (WildcardBndrInForallTelescope)

(Test case: testsuite/tests/rename/should_fail/T23501_fail.hs)

See the new Notes:

• Note [Type variable binders]
• Note [Wildcard binders in disallowed contexts]

To accommodate the new forms of binders, HsTyVarBndr was changed as follows (demonstrated without x-fields for clarity)

-- BEFORE (ignoring x-fields and locations)
data HsTyVarBndr flag
  = UserTyVar   flag Name
  | KindedTyVar flag Name HsKind

-- AFTER (ignoring x-fields and locations)
data HsTyVarBndr flag = HsTvb flag HsBndrVar HsBndrKind
data HsBndrVar  = HsBndrVar Name | HsBndrWildCard
data HsBndrKind = HsBndrNoKind | HsBndrKind LHsKind

The rest of the patch is downstream from this change.

To avoid a breaking change to the TH AST, we generate fresh names to replace wildcard binders instead of adding a dedicated representation for them (as discussed in #641).

And to put a cherry on top of the cake, we now allow wildcards in kind-polymorphic type variable binders in constructor patterns, see Note [Type patterns: binders and unifiers] and the tyPatToBndr function in GHC.Tc.Gen.HsType; example:

fn (MkT @(_ :: forall k. k -> Type) _ _) = ...

(Test case: testsuite/tests/typecheck/should_compile/T23501b.hs)