gccrs: fix ICE on generic type aliases

A type alias with generic parameters never resolved those parameters
into the type context, so the reachability pass asserted when looking
up their types. Resolve the generic parameters, as the other items do.

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-item.cc (TypeCheckItem::visit): Resolve
	the generic parameters of a type alias.

gcc/testsuite/ChangeLog:

	* rust/compile/type-alias-generic-params.rs: New test.

Signed-off-by: Lucas Ly Ba <lucas.ly-ba@outlook.com>