gccrs: Support generic constant impl items

Impl items can have constants defined which could in turn be generic this was
not supported by gccrs and missed. So for example:

  impl<T> Foo<T> {
    const MAGIC: usize = mem::size_of::<T>();
  }

This is a normal type parameter but in order to setup the generics we need to
create a synthetic TyTy::FnType so we can bind the parent's impl generics to
the type system and it just works like any other generic item at that point.
Then for example we have:

  impl<const N: usize> Foo<N> {
    const VALUE: usize = N;
  }

Again we consistently bind the this const generic parameter the same way so
the lazy evaluation of the generic can take place.

gcc/rust/ChangeLog:

	* backend/rust-compile-item.cc (CompileItem::visit): support the synthetic function consts
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::Resolve): likewise
	* typecheck/rust-hir-type-check-implitem.cc (TypeCheckImplItem::visit): create the synth
	* typecheck/rust-tyty.h: new flag for synthetic constant

gcc/testsuite/ChangeLog:

	* rust/execute/torture/const-generics-5.rs: New test.
	* rust/execute/torture/const-generics-6.rs: New test.
	* rust/execute/torture/const-generics-7.rs: New test.

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

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