gccrs: Fix small FixMe task in rust macro builtins

gccrs: Fix small FixMe task in rust macro builtins

gcc/rust/ChangeLog:

	* expand/rust-macro-builtins.cc: Change BuiltinMacro in
	builtin_macro_from_string to tl::optional<>
	* expand/rust-macro-builtins.h (enum class): Change BuiltinMacro
	in builtin_macro_from_string to tl::optional<>
	* resolve/rust-early-name-resolver.cc (EarlyNameResolver::visit):
	Resolved wrong type dependency of builtin_macro_from_string

Signed-off-by: jjasmine <tanghocle456@gmail.com>