Reapply "Add Chrono.h - std::chrono support header"

This is a resubmission of r284590. The mingw build should be fixed now. The
problem was we were matching time_t with _localtime_64s, which was incorrect on
_USE_32BIT_TIME_T systems. Instead I use localtime_s, which should always
evaluate to the correct function.

llvm-svn: 284720