gccrs: Add initial support for const generics

In order to support const generics we map the declarations to a ConstType this
means we reuse all our existing type coercion, unification code and generic
substitutions code to support this with minimal impact.

This patch adds support for:

1. Default const generics
2. Infer const generics
3. ADTType suport
4. unconstrained checks
5. ensure types of the const generic and default
6. validation if passing a const argument to a type argument

Lots of test cases now work and new ones added. More work is needed to support this
on functions and method resolution of impls like Foo<1> vs Foo<2> once thats in we
can look to support some of the const generic array impls next.

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc: useful debug
	* backend/rust-compile-stmt.cc (CompileStmt::visit): likewise
	* backend/rust-compile-type.cc (TyTyResolveCompile::visit): fold the capacity into ConstType
	* hir/tree/rust-hir-generic-param.h: make const
	* hir/tree/rust-hir-path.h: take into account const arguments now
	* typecheck/rust-hir-type-check-base.cc (TypeCheckBase::resolve_literal): needs const
	* typecheck/rust-hir-type-check-base.h: add error handling for const supported locations
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): const type the arrays
	* typecheck/rust-hir-type-check-implitem.cc (TypeCheckTopLevelExternItem::visit): update
	(TypeCheckImplItem::visit): likewise
	* typecheck/rust-hir-type-check-item.cc (TypeCheckItem::visit): likewise
	(TypeCheckItem::resolve_impl_block_substitutions): likewise
	* typecheck/rust-hir-type-check-pattern.cc (TypeCheckPattern::visit): wrap up const type
	* typecheck/rust-hir-type-check-type.cc (TypeCheckType::visit): likewise
	(TypeResolveGenericParam::visit): likewise
	(TypeResolveGenericParam::apply_trait_bounds): remove HIR::Generic from Param
	* typecheck/rust-hir-type-check.cc (TraitItemReference::get_type_from_fn): cleanup
	* typecheck/rust-tyty-subst.cc (SubstitutionParamMapping::SubstitutionParamMapping):
	handle const generics
	(SubstitutionParamMapping::get_type_representation): likewise
	(SubstitutionParamMapping::param_has_default_ty): likewise
	(SubstitutionParamMapping::get_default_ty): likewise
	(SubstitutionRef::infer_substitions): likewise
	* typecheck/rust-tyty-subst.h: likewise
	* typecheck/rust-tyty-util.cc (TyVar::get_implicit_const_infer_var): new helper
	* typecheck/rust-tyty-util.h (class ConstType): likewise
	* typecheck/rust-tyty.cc (BaseType::is_concrete): check for array const concrete
	(ArrayType::as_string): update to const
	(ArrayType::handle_substitions): likewise
	(ParamType::ParamType): likewise
	(ParamType::get_generic_param): likewise
	(ParamType::clone): likewise
	(ConstType::ConstType): likewise
	(ConstType::set_value): likewise
	(ConstType::clone): likewise
	(ConstType::get_generic_param): likewise
	(generate_tree_str): new helper to pretty print gimple
	(ConstType::get_name): uses the generate_tree_str
	(ConstType::handle_substitions): handle const infer's
	* typecheck/rust-tyty.h (RUST_TYTY): likewise
	* typecheck/rust-unify.cc (UnifyRules::expect_array): likewise
	(UnifyRules::expect_const): likewise

gcc/testsuite/ChangeLog:

	* rust/compile/const_generics_3.rs: this works now
	* rust/compile/const_generics_5.rs: likewise
	* rust/compile/const_generics_8.rs: move the failure to another test case
	* rust/compile/const_generics_10.rs: New test.
	* rust/compile/const_generics_11.rs: New test.
	* rust/compile/const_generics_12.rs: New test.
	* rust/compile/const_generics_13.rs: New test.
	* rust/compile/const_generics_14.rs: New test.
	* rust/compile/const_generics_15.rs: New test.
	* rust/compile/const_generics_16.rs: New test.
	* rust/compile/const_generics_9.rs: New test.

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

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