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

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.

Differential Revision: https://reviews.llvm.org/D108297

Author: ldionne

libcxx/include/__config

@@ -354,6 +354,16 @@
 #  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
@@ -408,6 +418,7 @@
 #    endif
 #  endif // __APPLE__
 #endif
+#endif // __has_attribute(using_if_exists)
 
 #ifndef _LIBCPP_CXX03_LANG
 # define _LIBCPP_ALIGNOF(_Tp) alignof(_Tp)

libcxx/include/ctime

@@ -59,7 +59,7 @@ int timespec_get( struct timespec *ts, int base); // C++17
 // 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