PR c++/80265 - constexpr __builtin_mem*.

The library has already worked around this issue, but I was curious about
why it wasn't working.  The answer: because we were passing &var to fold,
which doesn't know about the constexpr values hash table.  Fixed by passing
&"str" instead.

	* constexpr.c (cxx_eval_builtin_function_call): Expose STRING_CST
	to str/mem builtins.