gccrs: ast: Add ExternalTypeItem node

gccrs: ast: Add ExternalTypeItem node

gcc/rust/ChangeLog:

	* ast/rust-item.h (class ExternalTypeItem): New class.
	* ast/rust-ast.cc (ExternalTypeItem::as_string): New function.
	(ExternalTypeItem::accept_vis): Likewise.
	* ast/rust-ast-full-decls.h (class ExternalTypeItem): Declare class.
	* ast/rust-ast-dump.cc (Dump::visit): Add base visitor for ExternalTypeItem.
	* ast/rust-ast-dump.h: Likewise.
	* ast/rust-ast-visitor.h: Likewise.
	* checks/errors/rust-feature-gate.h: Likewise.
	* expand/rust-attribute-visitor.cc (AttrVisitor::visit): Likewise.
	* expand/rust-attribute-visitor.h: Likewise.
	* hir/rust-ast-lower-base.cc (ASTLoweringBase::visit): Likewise.
	* hir/rust-ast-lower-base.h: Likewise.
	* resolve/rust-ast-resolve-base.cc (ResolverBase::visit): Likewise.
	* resolve/rust-ast-resolve-base.h: Likewise.
	* resolve/rust-early-name-resolver.cc (EarlyNameResolver::visit): Likewise.
	* resolve/rust-early-name-resolver.h: Likewise.
	* util/rust-attributes.cc (AttributeChecker::visit): Likewise.
	* util/rust-attributes.h: Likewise.