Fix test for __is_aggregate support in Clang

2017-08-09  Katsuhiko Nishimra  <ktns.87@gmail.com>

	* include/std/type_traits (_GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE): Use
	__is_identifier instead of __has_builtin.

From-SVN: r251008

libstdc++-v3/ChangeLog

+2017-08-09  Katsuhiko Nishimra  <ktns.87@gmail.com>
+
+	* include/std/type_traits (_GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE): Use
+	__is_identifier instead of __has_builtin.
+
 2017-08-09  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/79820

libstdc++-v3/include/std/type_traits

@@ -2894,9 +2894,9 @@ template <typename _From, typename _To>
 
 #if __GNUC__ >= 7
 # define _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE 1
-#elif defined __has_builtin
+#elif defined(__is_identifier)
 // For non-GNU compilers:
-# if __has_builtin(__is_aggregate)
+# if ! __is_identifier(__is_aggregate)
 #  define _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE 1
 # endif
 #endif