gccrs: hir: Add ExternalTypeItem node

gccrs: hir: Add ExternalTypeItem node

gcc/rust/ChangeLog:

	* hir/tree/rust-hir-item.h (class ExternalTypeItem): New class.
	* hir/tree/rust-hir.cc (ExternalTypeItem::as_string): Likewise.
	* backend/rust-compile-extern.h: Add base for handling HIR::ExternalTypeItem
	node.
	* checks/errors/borrowck/rust-bir-builder-struct.h: Likewise.
	* checks/errors/borrowck/rust-function-collector.h: Likewise.
	* checks/errors/rust-const-checker.cc (ConstChecker::visit): Likewise.
	* checks/errors/rust-const-checker.h: Likewise.
	* checks/errors/rust-unsafe-checker.cc (UnsafeChecker::visit): Likewise.
	* checks/errors/rust-unsafe-checker.h: Likewise.
	* hir/rust-ast-lower-extern.h: Likewise.
	* hir/rust-hir-dump.cc (Dump::visit): Likewise.
	* hir/rust-hir-dump.h: Likewise.
	* hir/tree/rust-hir-full-decls.h (class ExternalTypeItem): Likewise.
	* hir/tree/rust-hir-visitor.h: Likewise.
	(ExternalTypeItem::accept_vis): Likewise.
	* typecheck/rust-hir-type-check-implitem.cc (TypeCheckTopLevelExternItem::visit): Likewise.
	* typecheck/rust-hir-type-check-implitem.h: Likewise.