Fix most GCC attribute ignored warnings

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@267074 91177308-0d34-0410-b5e6-96231b3b80d8

include/stdexcept

@@ -53,7 +53,11 @@ public:
 #ifndef _LIBCPP___REFSTRING
 _LIBCPP_BEGIN_NAMESPACE_STD
 class _LIBCPP_HIDDEN __libcpp_refstring {
-    const char *__imp_ _LIBCPP_UNUSED;
+#ifdef __clang__
+    const char *__imp_ __attribute__((__unused__)); // only clang emits a warning
+#else
+    const char *__imp_;
+#endif
 };
 _LIBCPP_END_NAMESPACE_STD
 #endif