chrono: correct the units for the epoch bias

As pointed out by Howard, this is actually 134774 days (* 24 * 3600),
and therefore seconds, not 100ns units.  Adjust the units to reflect
reality.

llvm-svn: 290824

Author: compnerd

libcxx/src/chrono.cpp

@@ -53,8 +53,7 @@ system_clock::now() _NOEXCEPT
                                                     nanoseconds::period>>;
 
   // The Windows epoch is Jan 1 1601, the Unix epoch Jan 1 1970.
-  static _LIBCPP_CONSTEXPR const filetime_duration
-      nt_to_unix_epoch{11644473600};
+  static _LIBCPP_CONSTEXPR const seconds nt_to_unix_epoch{11644473600};
 
   FILETIME ft;
 #if _WIN32_WINNT >= _WIN32_WINNT_WIN8