Fix TimeValue::toWin32Time() to be symmetric to fromWin32Time() and compatible to Win32's FILETIME.

llvm-ar is the only user of toWin32Time() (via setLastModificationAndAccessTime), and r186298 can be reverted.
It had been buggy since the initial commit.

FIXME: Could we rename {from|to}Win32Time as {from|to}Win32FILETIME in TimeValue?
unittests/Support: Add TimeValue.Win32FILETIME, corresponding to r186374.