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

llvm-svn: 228281

libcxx/include/__config

@@ -258,6 +258,8 @@ typedef __char32_t char32_t;
 #  define _LIBCPP_NORETURN __attribute__ ((noreturn))
 #endif
 
+#define _LIBCPP_UNUSED __attribute__((__unused__))
+
 #if !(__has_feature(cxx_defaulted_functions))
 #define _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
 #endif  // !(__has_feature(cxx_defaulted_functions))
@@ -380,6 +382,8 @@ namespace std {
 
 #define _LIBCPP_NORETURN __attribute__((noreturn))
 
+#define _LIBCPP_UNUSED __attribute__((__unused__))
+
 #if _GNUC_VER >= 407
 #define _LIBCPP_UNDERLYING_TYPE(T) __underlying_type(T)
 #define _LIBCPP_IS_LITERAL(T) __is_literal_type(T)
@@ -477,6 +481,7 @@ using namespace _LIBCPP_NAMESPACE __attribute__((__strong__));
 #define _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
 #define __alignof__ __alignof
 #define _LIBCPP_NORETURN __declspec(noreturn)
+#define _LIBCPP_UNUSED
 #define _ALIGNAS(x) __declspec(align(x))
 #define _LIBCPP_HAS_NO_VARIADICS
 
@@ -500,6 +505,7 @@ namespace std {
 #define _ALIGNAS_TYPE(x) __attribute__((__aligned__(__alignof(x))))
 #define _ATTRIBUTE(x) __attribute__((x))
 #define _LIBCPP_NORETURN __attribute__((noreturn))
+#define _LIBCPP_UNUSED
 
 #define _NOEXCEPT throw()
 #define _NOEXCEPT_(x)

libcxx/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