lang-items: Make lang items enum stronger, rename class, cleanup ns.

lang-items: Make lang items enum stronger, rename class, cleanup ns.

gcc/rust/ChangeLog:

	* util/rust-lang-item.h (class RustLangItem): Renamed to...
	(class LangItem): ...this. Rename ItemType enum to Kind
	* util/rust-lang-item.cc: Rename methods to use new class name.
	* backend/rust-compile-expr.cc (CompileExpr::visit): Use new lang-item API.
	(CompileExpr::resolve_operator_overload): Likewise.
	* backend/rust-compile-expr.h: Likewise.
	* hir/rust-ast-lower-base.cc (ASTLoweringBase::handle_lang_item_attribute): Likewise.
	* typecheck/rust-autoderef.cc (Adjuster::try_deref_type): Likewise.
	(AutoderefCycle::cycle): Likewise.
	* typecheck/rust-autoderef.h: Likewise.
	* typecheck/rust-hir-type-bounds.h: Likewise.
	* typecheck/rust-hir-type-check-base.cc (TypeCheckBase::get_marker_predicate): Likewise.
	* typecheck/rust-hir-type-check-base.h: Likewise.
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): Likewise.
	* typecheck/rust-hir-type-check-expr.h: Likewise.
	* typecheck/rust-hir-type-check-type.cc (TypeResolveGenericParam::visit): Likewise.
	* typecheck/rust-tyty-bounds.cc (TypeBoundsProbe::assemble_sized_builtin): Likewise.
	(TypeBoundsProbe::assemble_builtin_candidate): Likewise.
	(TypeCheckBase::get_predicate_from_bound): Likewise.
	* typecheck/rust-tyty.cc (ClosureType::setup_fn_once_output): Likewise.
	* util/rust-hir-map.cc (Mappings::get_lang_item): Likewise.
	(Mappings::lookup_trait_item_lang_item): Likewise.
	* util/rust-hir-map.h: Likewise.