clean up use of _WIN32

Replace the use of _WIN32 in libc++. Replace most use with a C runtime
check _LIBCPP_MSVCRT or the new _LIBCPP_WIN32 to indicate that we are
using the Win32 API. Use a new _LIBCPP_WCHAR_IS_UCS2 to indicate that we
are on an environment that has a short wchar_t.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290910 91177308-0d34-0410-b5e6-96231b3b80d8

src/chrono.cpp

@@ -12,7 +12,7 @@
 #include "system_error"  // __throw_system_error
 #include <time.h>        // clock_gettime, CLOCK_MONOTONIC and CLOCK_REALTIME
 
-#if defined(_WIN32)
+#if defined(_LIBCPP_WIN32API)
 #define WIN32_LEAN_AND_MEAN
 #define VC_EXTRA_LEAN
 #include <Windows.h>
@@ -22,13 +22,13 @@
 #else
 #if !defined(CLOCK_REALTIME)
 #include <sys/time.h>        // for gettimeofday and timeval
-#endif
-#endif
+#endif // !defined(CLOCK_REALTIME)
+#endif // defined(_LIBCPP_WIN32API)
 
 #if !defined(_LIBCPP_HAS_NO_MONOTONIC_CLOCK)
 #if __APPLE__
 #include <mach/mach_time.h>  // mach_absolute_time, mach_timebase_info_data_t
-#elif !defined(_WIN32) && !defined(CLOCK_MONOTONIC)
+#elif !defined(_LIBCPP_WIN32API) && !defined(CLOCK_MONOTONIC)
 #error "Monotonic clock not implemented"
 #endif
 #endif
@@ -45,7 +45,7 @@ const bool system_clock::is_steady;
 system_clock::time_point
 system_clock::now() _NOEXCEPT
 {
-#if defined(_WIN32)
+#if defined(_LIBCPP_WIN32API)
   // FILETIME is in 100ns units
   using filetime_duration =
       _VSTD::chrono::duration<__int64,
@@ -158,7 +158,7 @@ steady_clock::now() _NOEXCEPT
     return time_point(duration(fp()));
 }
 
-#elif defined(_WIN32)
+#elif defined(_LIBCPP_WIN32API)
 
 steady_clock::time_point
 steady_clock::now() _NOEXCEPT