Fix unused private field warning in stdexcept after r207695.

Add a new _LIBCPP_UNUSED define in __config, which can be used to
indicate explicitly unused items, and apply it to the __imp__ field of
__libcpp_refstring.

Somebody who knows about Microsoft C++ and IBM C++ should fill in the
unused attribute syntax appropriate for those compilers, if there is
any.

Differential Revision: http://reviews.llvm.org/D6836


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@228281 91177308-0d34-0410-b5e6-96231b3b80d8

include/stdexcept

@@ -53,7 +53,7 @@ public:
 #ifndef _LIBCPP___REFSTRING
 _LIBCPP_BEGIN_NAMESPACE_STD
 class _LIBCPP_HIDDEN __libcpp_refstring {
-    const char *__imp_;
+    const char *__imp_ _LIBCPP_UNUSED;
 };
 _LIBCPP_END_NAMESPACE_STD
 #endif