gccrs: ast: Only expand expressions and types if the kind is right

gccrs: ast: Only expand expressions and types if the kind is right

gcc/rust/ChangeLog:

	* ast/rust-ast.h: Add assertions and accessors for fragment nodes.
	* expand/rust-attribute-visitor.cc (AttrVisitor::visit): Fix expansion
	context typo when visiting `InherentImpl` items.
	(AttrVisitor::maybe_expand_expr): Use new Fragment accessor to fetch
	properly typed node.
	(AttrVisitor::maybe_expand_type): Likewise.
	* expand/rust-macro-expand.cc (transcribe_type): Emit parse errors
	when trying to parse a type.