Make GADT constructors adhere to the forall-or-nothing rule properly

Issue #18191 revealed that the types of GADT constructors don't quite
adhere to the `forall`-or-nothing rule. This patch serves to clean up
this sad state of affairs somewhat. The main change is not in the
code itself, but in the documentation, as this patch introduces two
sections to the GHC User's Guide:

* A "Formal syntax for GADTs" section that presents a BNF-style
  grammar for what is and isn't allowed in GADT constructor types.
  This mostly exists to codify GHC's existing behavior, but it also
  imposes a new restriction that addresses #18191: the outermost
  `forall` and/or context in a GADT constructor is not allowed to be
  surrounded by parentheses. Doing so would make these
  `forall`s/contexts nested, and GADTs do not support nested
  `forall`s/contexts at present.

* A "`forall`-or-nothing rule" section that describes exactly what
  the `forall`-or-nothing rule is all about. Surprisingly, there was
  no mention of this anywhere in the User's Guide up until now!

To adhere the new specification in the "Formal syntax for GADTs"
section of the User's Guide, the following code changes were made:

* A new function, `GHC.Hs.Type.splitLHsGADTPrefixTy`, was introduced.
  This is very much like `splitLHsSigmaTy`, except that it avoids
  splitting apart any parentheses, which can be syntactically
  significant for GADT types. See
  `Note [No nested foralls or contexts in GADT constructors]` in
  `GHC.Hs.Type`.

* `ConDeclGADTPrefixPs`, an extension constructor for `XConDecl`, was
  introduced so that `GHC.Parser.PostProcess.mkGadtDecl` can return
  it when given a prefix GADT constructor. Unlike `ConDeclGADT`,
  `ConDeclGADTPrefixPs` does not split the GADT type into its argument
  and result types, as this cannot be done until after the type is
  renamed (see `Note [GADT abstract syntax]` in `GHC.Hs.Decls` for why
  this is the case).

* `GHC.Renamer.Module.rnConDecl` now has an additional case for
  `ConDeclGADTPrefixPs` that (1) splits apart the full `LHsType` into
  its `forall`s, context, argument types, and result type, and
  (2) checks for nested `forall`s/contexts. Step (2) used to be
  performed the typechecker (in `GHC.Tc.TyCl.badDataConTyCon`) rather
  than the renamer, but now the relevant code from the typechecker
  can simply be deleted.

  One nice side effect of this change is that we are able to give a
  more accurate error message for GADT constructors that use visible
  dependent quantification (e.g., `MkFoo :: forall a -> a -> Foo a`),
  which improves the stderr in the `T16326_Fail6` test case.

Fixes #18191. Bumps the Haddock submodule.