gccrs: Initial support for Return Position Impl Trait

This is the initial patch for RPIT, we can build on this to handle the
more complex cases but there are enough distinct changes going on here
that it should just get merged now.

RPIT is really a sneaky generic so for example:

  fn foo() -> impl Bar {
      Baz
  }

This is represented as: fn () -> OpaqueType Bar. But when we handle the
coercion site for Baz on impl Bar when we type resolve the function we
know that the underlying type  is Baz. Note this function is _not_ generic
so its using this special OpaqueType and keeping track of the underlying type
in its ty_ref reference hir-id which will resolve to Baz.

This also means if we have a case where maybe this was in an if statement:

  fn foo(a: i32) -> impl Bar {
      if a > 10 {
        Baz
      } else {
        Qux
      }
  }

The rules of impl Bar is that Baz is handled but Baz and Qux are different
underlying types so this is not allowed. The reason is impl traits are not
generic and although from a programmer perspective the callers dont know what
the underlying type is, the compiler _knows_ what it is. So really when
you call a function and get its return position impl trait the compiler knows
what to do and does all whats nessecary to handle calling functions using that
type etc.

gcc/rust/ChangeLog:

	* backend/rust-compile-type.cc (TyTyResolveCompile::visit): we need to resolve the
	underlying type
	* typecheck/rust-substitution-mapper.cc (SubstMapperInternal::visit): just clone
	* typecheck/rust-tyty-call.cc (TypeCheckCallExpr::visit):
	ensure we monomphize to get the underlying
	* typecheck/rust-tyty.cc (BaseType::destructure): handle opaque types
	(OpaqueType::resolve): this is much simpler now
	(OpaqueType::handle_substitions): no longer needed
	* typecheck/rust-tyty.h: update header
	* typecheck/rust-unify.cc (UnifyRules::expect_opaque): unify rules for opaque

gcc/testsuite/ChangeLog:

	* rust/compile/bad-rpit1.rs: New test.
	* rust/execute/torture/impl_rpit1.rs: New test.
	* rust/execute/torture/impl_rpit2.rs: New test.
	* rust/execute/torture/impl_rpit3.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

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