re PR ipa/86590 (Codegen is poor when passing std::string by value with _GLIBCXX_EXTERN_TEMPLATE undefined)

	PR libstdc++/86590
	* include/bits/char_traits.h (__constant_string_p,
	__constant_char_array_p): Use __builtin_is_constant_evaluated if
	available.

From-SVN: r268112