[libc++] [test] C++14/17-friendly TEST_IS_CONSTANT_EVALUATED macro.

Reviewed as part of D114658.
Ultimately this will probably have to be flipped around and renamed
`TEST_IS_RUNTIME`, and extended with `TEST_IS_RUNTIME_OR_CXX20` (once
constexpr std::string support is added) and so on for every new C++
version. But we don't need that flexibility yet, so we're not adding it.

libcxx/test/support/test_macros.h

@@ -146,9 +146,11 @@
 #endif
 
 #if defined(__cpp_lib_is_constant_evaluated) && __cpp_lib_is_constant_evaluated >= 201811L
-#  define TEST_IS_CONSTANT_EVALUATED std::is_constant_evaluated()
+# define TEST_IS_CONSTANT_EVALUATED std::is_constant_evaluated()
+#elif __has_builtin(__builtin_is_constant_evaluated)
+# define TEST_IS_CONSTANT_EVALUATED __builtin_is_constant_evaluated()
 #else
-#  define TEST_IS_CONSTANT_EVALUATED false
+# define TEST_IS_CONSTANT_EVALUATED false
 #endif
 
 #if TEST_STD_VER >= 14