[libc++] Fix __verbose_abort in C++11

[ldionne]

Summary

__use() can't be marked as constexpr in C++11 because it returns void,
which is not a literal type. But it turns out that we just don't need
to define it at all, since it's never called outside of decltype.

libcxx/include/__verbose_abort

 // to move to customizing _LIBCPP_VERBOSE_ABORT instead.
 #  if defined(_LIBCPP_AVAILABILITY_HAS_NO_VERBOSE_ABORT) && defined(_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED)
 #    undef _LIBCPP_AVAILABILITY_HAS_NO_VERBOSE_ABORT
 #    warning _LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED is deprecated, please customize _LIBCPP_VERBOSE_ABORT instead
 #  endif
 
 #  if defined(_LIBCPP_AVAILABILITY_HAS_NO_VERBOSE_ABORT)
 // The decltype is there to suppress -Wunused warnings in this configuration.
-_LIBCPP_HIDE_FROM_ABI inline _LIBCPP_CONSTEXPR void __use(const char*, ...) { }
+void __use(const char*, ...);
 #    define _LIBCPP_VERBOSE_ABORT(...) (decltype(::std::__use(__VA_ARGS__))(), __builtin_abort())
 #  else
 #    define _LIBCPP_VERBOSE_ABORT(...) ::std::__libcpp_verbose_abort(__VA_ARGS__)
 #  endif
 
 #endif // !defined(_LIBCPP_VERBOSE_ABORT)
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___VERBOSE_ABORT