gccrs: Add support for initial generic associated types

[philberty]

NOTE: Gat's are not finished this is just a starting point

This patch is the initial part in supporting generic associated types. In rust we have trait item types that get implemented for example:

  trait Foo<T> {
    type Bar
  }

  impl<T> Foo for T {
    type Bar = T
  }

The trait position uses a Ty::Placeholder which is just a thing that gets set for lazy evaluation to the impl type alias which is actually a Ty::Projection see:

0798add

For more info the projection type needs to hold onto generics in order to properly support generic types this GAT's support extends this all the way to the placeholder which still needs to be done.

Fixes #4276

gcc/rust/ChangeLog:

* ast/rust-ast.cc (TraitItemType::as_string): add generic params
* ast/rust-ast.h: remove old comment
* ast/rust-item.h: add generic params to associated type
* ast/rust-type.h: remove old comment
* hir/rust-ast-lower-implitem.cc (ASTLowerTraitItem::visit): hir lowering for gat's
* hir/tree/rust-hir-item.cc (TraitItemType::TraitItemType): gat's on TraitItemType
(TraitItemType::operator=): preserve generic params
* hir/tree/rust-hir-item.h: likewise
* hir/tree/rust-hir.cc (TraitItemType::as_string): likewise
* parse/rust-parse-impl.h (Parser::parse_trait_type): hit the < and parse params
* typecheck/rust-hir-type-check-implitem.cc (TypeCheckImplItemWithTrait::visit): typecheck
* typecheck/rust-tyty.cc (BaseType::has_substitutions_defined): dont destructure

gcc/testsuite/ChangeLog:

* rust/compile/gat1.rs: New test.
* rust/execute/torture/gat1.rs: New test.

*Please write a comment explaining your change. This is the message
that will be part of the merge commit.

[CohenArthur]

LGTM!