Fix unused private field warning in stdexcept after r207695

[dim]

Summary

As I mentioned on IRC, r207695 leads to a warning when compiling <stdexcept> (or any other header that includes it), if you use -Wsystem-headers, like we do in FreeBSD:

In file included from test-stdexcept.cpp:1:
In file included from include/array:109:
include/stdexcept:56:17: warning: private field '__imp_' is not used [-Wunused-private-field]
    const char *__imp_;
                ^
1 warning generated.

Apart from just including <refstring> in stdexcept, which is apparently too costly, the fix with lowest impact would probably be to add attribute((unused)) to the imp_ field.

However, there isn't any _LIBCPP_UNUSED define in __config yet, so that would have to be added, like in this proposed change.

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

[dim]

Marshall? Are you OK with this? :)

[dim]

Ping.

[EricWF]

LGTM as well. You can commit this. I'll let Marshall know.

libcxx/trunk/include/__config

 #endif
 
 #if __has_feature(cxx_attributes)
 #  define _LIBCPP_NORETURN [[noreturn]]
 #else
 #  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))
 
 #if !(__has_feature(cxx_deleted_functions))
 #define _LIBCPP_HAS_NO_DELETED_FUNCTIONS
 #endif  // !(__has_feature(cxx_deleted_functions))
 

 
 #elif defined(__GNUC__)
 
 #define _ALIGNAS(x) __attribute__((__aligned__(x)))
 #define _ALIGNAS_TYPE(x) __attribute__((__aligned__(__alignof(x))))
 
 #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)
 #endif
 
 #if !__EXCEPTIONS
 #define _LIBCPP_NO_EXCEPTIONS
 #endif

 #define _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
 #define _LIBCPP_HAS_NO_CONSTEXPR
 #define _LIBCPP_HAS_NO_CXX14_CONSTEXPR
 #define _LIBCPP_HAS_NO_UNICODE_CHARS
 #define _LIBCPP_HAS_NO_DELETED_FUNCTIONS
 #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
 
 #define _NOEXCEPT throw ()
 #define _NOEXCEPT_(x)
 #define _NOEXCEPT_OR_FALSE(x) false
 
 #define _LIBCPP_BEGIN_NAMESPACE_STD namespace std {
 #define _LIBCPP_END_NAMESPACE_STD  }
 #define _VSTD std
 
 #  define _LIBCPP_WEAK
 namespace std {
 }
 
 #define _LIBCPP_HAS_NO_ASAN
 
 #elif defined(__IBMCPP__)
 
 #define _ALIGNAS(x) __attribute__((__aligned__(x)))
 #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)
 #define _NOEXCEPT_OR_FALSE(x) false
 
 #define _LIBCPP_HAS_NO_TEMPLATE_ALIASES
 #define _LIBCPP_HAS_NO_ADVANCED_SFINAE
 #define _LIBCPP_HAS_NO_ALWAYS_INLINE_VARIADICS

libcxx/trunk/include/stdexcept

 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 #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
 
 namespace std  // purposefully not using versioning namespace
 {
 
 class _LIBCPP_EXCEPTION_ABI logic_error