macro: Allow builtin `MacroInvocation`s within the AST

macro: Allow builtin `MacroInvocation`s within the AST

gcc/rust/ChangeLog:

	* ast/rust-macro.h (enum class): Add `BuiltinMacro` enum class.
	* expand/rust-attribute-visitor.cc (AttrVisitor::visit): Mention
	switching on `macro.kind` once builtin macro invocations are properly
	handled.
	* parse/rust-parse-impl.h (Parser::parse_macro_invocation): Switch to new MacroInvocation
	API.
	(Parser::parse_type): Likewise.
	(Parser::parse_type_no_bounds): Likewise.