[libc++] Fix clock selection in chrono.cpp and filesystem_clock.cpp

[ldionne]

Summary

This patch partly reverts the change that was made in 5f1ba3a502
regarding the clock selection on Apple platforms. It turns out that
gettimeofday() is marked as obsolete by POSIX and clock_gettime() is
recommended instead. Since both are equivalent for our purposes,
prefer using clock_gettime() even on Apple platforms.

[ldionne]

Back-deployment failures are known flukes unrelated to this patch.

libcxx/src/chrono.cpp

 #if __has_include(<unistd.h>)
 # include <unistd.h> // _POSIX_TIMERS
 #endif
 
 #if __has_include(<sys/time.h>)
 # include <sys/time.h> // for gettimeofday and timeval
 #endif
 
-#if !defined(__APPLE__) && defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0
-# define _LIBCPP_USE_CLOCK_GETTIME
+#if defined(__APPLE__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
+# define _LIBCPP_HAS_CLOCK_GETTIME
 #endif
 
 #if defined(_LIBCPP_WIN32API)
 #  define WIN32_LEAN_AND_MEAN
 #  define VC_EXTRA_LEAN
 #  include <windows.h>
 #  if _WIN32_WINNT >= _WIN32_WINNT_WIN8
 #    include <winapifamily.h>
#else
   GetSystemTimeAsFileTimeFunc.fp(&ft);
 #endif
 
   filetime_duration d{(static_cast<__int64>(ft.dwHighDateTime) << 32) |
                        static_cast<__int64>(ft.dwLowDateTime)};
   return system_clock::time_point(duration_cast<system_clock::duration>(d - nt_to_unix_epoch));
 }
 
-#elif defined(_LIBCPP_USE_CLOCK_GETTIME) && defined(CLOCK_REALTIME)
+#elif defined(_LIBCPP_HAS_CLOCK_GETTIME)
 
 static system_clock::time_point __libcpp_system_clock_now() {
   struct timespec tp;
   if (0 != clock_gettime(CLOCK_REALTIME, &tp))
     __throw_system_error(errno, "clock_gettime(CLOCK_REALTIME) failed");
   return system_clock::time_point(seconds(tp.tv_sec) + microseconds(tp.tv_nsec / 1000));
 }
 
static steady_clock::time_point __libcpp_steady_clock_now() noexcept {
   // Implicitly link against the vDSO system call ABI without
   // requiring the final link to specify -lzircon explicitly when
   // statically linking libc++.
 #    pragma comment(lib, "zircon")
 
   return steady_clock::time_point(nanoseconds(_zx_clock_get_monotonic()));
 }
 
-#  elif defined(_LIBCPP_USE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC)
+#  elif defined(_LIBCPP_HAS_CLOCK_GETTIME)
 
 static steady_clock::time_point __libcpp_steady_clock_now() {
     struct timespec tp;
     if (0 != clock_gettime(CLOCK_MONOTONIC, &tp))
         __throw_system_error(errno, "clock_gettime(CLOCK_MONOTONIC) failed");
     return steady_clock::time_point(seconds(tp.tv_sec) + nanoseconds(tp.tv_nsec));
 }
 

libcxx/src/filesystem/filesystem_clock.cpp

 #if __has_include(<unistd.h>)
 # include <unistd.h> // _POSIX_TIMERS
 #endif
 
 #if __has_include(<sys/time.h>)
 # include <sys/time.h> // for gettimeofday and timeval
 #endif
 
-#if !defined(__APPLE__) && defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0
-# define _LIBCPP_USE_CLOCK_GETTIME
+#if defined(__APPLE__) || (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0)
+# define _LIBCPP_HAS_CLOCK_GETTIME
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM
 
 const bool _FilesystemClock::is_steady;
 
 _FilesystemClock::time_point _FilesystemClock::now() noexcept {
   typedef chrono::duration<rep> __secs;
 #if defined(_LIBCPP_WIN32API)
   typedef chrono::duration<rep, nano> __nsecs;
   FILETIME time;
   GetSystemTimeAsFileTime(&time);
   detail::TimeSpec tp = detail::filetime_to_timespec(time);
   return time_point(__secs(tp.tv_sec) +
                     chrono::duration_cast<duration>(__nsecs(tp.tv_nsec)));
-#elif defined(_LIBCPP_USE_CLOCK_GETTIME) && defined(CLOCK_REALTIME)
+#elif defined(_LIBCPP_HAS_CLOCK_GETTIME)
   typedef chrono::duration<rep, nano> __nsecs;
   struct timespec tp;
   if (0 != clock_gettime(CLOCK_REALTIME, &tp))
     __throw_system_error(errno, "clock_gettime(CLOCK_REALTIME) failed");
   return time_point(__secs(tp.tv_sec) +
                     chrono::duration_cast<duration>(__nsecs(tp.tv_nsec)));
 #else
   typedef chrono::duration<rep, micro> __microsecs;
   timeval tv;
   gettimeofday(&tv, 0);
   return time_point(__secs(tv.tv_sec) + __microsecs(tv.tv_usec));
-#endif // CLOCK_REALTIME
+#endif
 }
 
 _LIBCPP_END_NAMESPACE_FILESYSTEM