gccrs: Add ConstType boiler plate to handle const generics

gccrs: Add ConstType boiler plate to handle const generics

This patch is all about just putting in the boiler plate for the new
BaseGeneric TyTy::ConstType. Nothing is really change but just the
nessecary cogs added to the machine.

gcc/rust/ChangeLog:

	* backend/rust-compile-type.cc (TyTyResolveCompile::visit): error_mark_node for const types
	* backend/rust-compile-type.h: boilerplate
	* checks/errors/borrowck/rust-bir-fact-collector.h: likewise
	* checks/errors/borrowck/rust-bir-place.h: likewise
	* checks/errors/privacy/rust-privacy-reporter.cc (PrivacyReporter::check_base_type_privacy):
	likewise
	* typecheck/rust-substitution-mapper.cc (SubstMapperInternal::visit): likewise
	* typecheck/rust-substitution-mapper.h: likewise
	* typecheck/rust-tyty-bounds.cc (TypeBoundsProbe::assemble_marker_builtins): likewise
	* typecheck/rust-tyty-call.h: likewise
	* typecheck/rust-tyty-cmp.h (class ConstCmp): likewise
	* typecheck/rust-tyty-variance-analysis-private.h: likewise
	* typecheck/rust-tyty-visitor.h: likewise
	* typecheck/rust-tyty.cc (TypeKindFormat::to_string): likewise
	(BaseType::is_unit): likewise
	(BaseType::has_substitutions_defined): likewise
	(BaseType::needs_generic_substitutions): likewise
	(ConstType::ConstType): likewise
	(ConstType::accept_vis): likewise
	(ConstType::as_string): likewise
	(ConstType::can_eq): likewise
	(ConstType::clone): likewise
	(ConstType::get_symbol): likewise
	(ConstType::get_generic_param): likewise
	(ConstType::can_resolve): likewise
	(ConstType::resolve): likewise
	(ConstType::get_name): likewise
	(ConstType::is_equal): likewise
	(ConstType::handle_substitions): likewise
	* typecheck/rust-tyty.h (enum TypeKind): new tyty_kind
	(class ConstType): new type
	* typecheck/rust-unify.cc (UnifyRules::go): Handle a const type unify
	(UnifyRules::expect_inference_variable): likewise
	(UnifyRules::expect_adt): likewise
	(UnifyRules::expect_str): likewise
	(UnifyRules::expect_reference): likewise
	(UnifyRules::expect_pointer): likewise
	(UnifyRules::expect_param): likewise
	(UnifyRules::expect_array): likewise
	(UnifyRules::expect_slice): likewise
	(UnifyRules::expect_fndef): likewise
	(UnifyRules::expect_fnptr): likewise
	(UnifyRules::expect_tuple): likewise
	(UnifyRules::expect_bool): likewise
	(UnifyRules::expect_char): likewise
	(UnifyRules::expect_int): likewise
	(UnifyRules::expect_uint): likewise
	(UnifyRules::expect_float): likewise
	(UnifyRules::expect_isize): likewise
	(UnifyRules::expect_usize): likewise
	(UnifyRules::expect_placeholder): likewise
	(UnifyRules::expect_projection): likewise
	(UnifyRules::expect_dyn): likewise
	(UnifyRules::expect_closure): likewise
	(UnifyRules::expect_const): likewise
	* typecheck/rust-unify.h: new expect_const_type handler

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