[libc++] Skip logic for detecting C11 features when using_if_exists is supported

[ldionne]

Summary

In the future, we'll want to rely exclusively on using_if_exists for this
job, but for now, only rely on it when the compiler supports that attribute.
That removes the possibility for getting the logic wrong.

libcxx/include/__config

 #endif // !defined(_LIBCPP_LITTLE_ENDIAN) && !defined(_LIBCPP_BIG_ENDIAN)
 
 #if __has_attribute(__no_sanitize__) && !defined(_LIBCPP_COMPILER_GCC)
 #  define _LIBCPP_NO_CFI __attribute__((__no_sanitize__("cfi")))
 #else
 #  define _LIBCPP_NO_CFI
 #endif
 
+// If the compiler supports using_if_exists, pretend we have those functions and they'll
+// be picked up if the C library provides them.
+//
+// TODO: Once we drop support for Clang 12, we can assume the compiler supports using_if_exists
+//       for platforms that don't have a conforming C11 library, so we can drop this whole thing.
+#if __has_attribute(using_if_exists)
+# define _LIBCPP_HAS_TIMESPEC_GET
+# define _LIBCPP_HAS_QUICK_EXIT
+# define _LIBCPP_HAS_ALIGNED_ALLOC
+#else
 #if (defined(__ISO_C_VISIBLE) && (__ISO_C_VISIBLE >= 2011)) || __cplusplus >= 201103L
 #  if defined(__FreeBSD__)
 #    define _LIBCPP_HAS_ALIGNED_ALLOC
 #    define _LIBCPP_HAS_QUICK_EXIT
 #    if __FreeBSD_version >= 1300064 || \
        (__FreeBSD_version >= 1201504 && __FreeBSD_version < 1300000)
 #      define _LIBCPP_HAS_TIMESPEC_GET
 #    endif
#    if ((defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101500) || \
          (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 130000) || \
          (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ >= 130000) || \
          (defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ >= 60000))
 #      define _LIBCPP_HAS_ALIGNED_ALLOC
 #      define _LIBCPP_HAS_TIMESPEC_GET
 #    endif
 #  endif // __APPLE__
 #endif
+#endif // __has_attribute(using_if_exists)
 
 #ifndef _LIBCPP_CXX03_LANG
 # define _LIBCPP_ALIGNOF(_Tp) alignof(_Tp)
 #elif defined(_LIBCPP_COMPILER_CLANG_BASED)
 # define _LIBCPP_ALIGNOF(_Tp) _Alignof(_Tp)
 #else
 # error "We don't know a correct way to implement alignof(T) in C++03 outside of Clang"
 #endif

libcxx/include/ctime

 
 // FIXME:
 // Apple SDKs don't define ::timespec_get unconditionally in C++ mode. This
 // should be fixed in future SDKs, but for the time being we need to avoid
 // trying to use that declaration when the SDK doesn't provide it. Note that
 // we're detecting this here instead of in <__config> because we can't include
 // system headers from <__config>, since it leads to circular module dependencies.
 // This is also meant to be a very temporary workaround until the SDKs are fixed.
-#if defined(__APPLE__)
+#if defined(__APPLE__) && !__has_attribute(using_if_exists)
 #   include <sys/cdefs.h>
 #   if defined(_LIBCPP_HAS_TIMESPEC_GET) && (__DARWIN_C_LEVEL < __DARWIN_C_FULL)
 #       define _LIBCPP_HAS_TIMESPEC_GET_NOT_ACTUALLY_PROVIDED
 #   endif
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD