Add new macro _LIBCPP_BUILTIN_MEMCMP_ISCONSTEXPR to use in std::char_traits.

This tells whether or not the builtin function __builtin_memcmp is constexpr.
Only defined for clang 4.0 and later, and not true for any shipping version of Apple's clang.



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291773 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -396,6 +396,15 @@ namespace std {
 #define _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK __attribute__((__no_sanitize__("unsigned-integer-overflow")))
 #endif 
 
+// A constexpr version of __builtin_memcmp was added in clang 4.0
+#if __has_builtin(__builtin_memcmp)
+# ifdef __apple_build_version__
+// No shipping version of Apple's clang has constexpr __builtin_memcmp
+# elif __clang_major__ > 3
+#  define _LIBCPP_BUILTIN_MEMCMP_ISCONSTEXPR
+# endif
+#endif
+
 #elif defined(_LIBCPP_COMPILER_GCC)
 
 #define _ALIGNAS(x) __attribute__((__aligned__(x)))