c++: Avoid ICE when constant evaluation of __builtin_strchr fails.

If we can't change the argument to &"...", use the original arg instead of
the partially munged one.

	PR c++/93331 - ICE with __builtin_strchr.
	* constexpr.c (cxx_eval_builtin_function_call): Use the original
	argument if we didn't manage to extract a STRING_CST.