gccrs: Create Rust::GGC::Ident

gccrs: Create Rust::GGC::Ident

This should make it easier for us to hand identifiers off to the
back end.

gcc/rust/ChangeLog:

	* Make-lang.in (GRS_OBJS): Add rust-ggc.o.
	* backend/rust-compile-base.cc
	(HIRCompileBase::compile_function): Adjust call to
	Backend::function.
	(HIRCompileBase::compile_constant_item): Likewise and adjust
	initialization of Backend::typed_identifier.
	* backend/rust-compile-expr.cc (CompileExpr::visit): Adjust call
	to Backend::label.
	* backend/rust-compile-type.cc (TyTyResolveCompile::visit):
	Adjust initialization of Backend::typed_identifier.
	* rust-backend.h: Add includes.
	(Backend::GGC::Ident): Use Rust::GGC::Ident.
	(struct typed_identifier): Store name as a GGC::Ident rather
	than a std::string and adjust constructors.
	(named_type): Take GGC::Ident/tl::optional<GGC::Ident> rather
	than std::string.
	(global_variable): Likewise.
	(local_variable): Likewise.
	(parameter_variable): Likewise.
	(static_chain_variable): Likewise.
	(label): Likewise.
	(function): Likewise.
	* rust-gcc.cc (named_type): Likewise.
	(global_variable): Likewise.
	(local_variable): Likewise.
	(parameter_variable): Likewise.
	(static_chain_variable): Likewise.
	(label): Likewise.
	(function): Likewise.
	(function_defer_statement): Adjust call to Backend::label.
	(get_identifier_from_string): Remove function.
	(fill_in_fields): Handle adjustments to typed_identifier.
	* util/rust-ggc.cc: New file.
	* util/rust-ggc.h: New file.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>