chrono: implement a Windows version of system_clock::now

system_clock::now is not entirely straight forward on Windows, which
does not have a clock_gettime function.

GetSystemTimeAsFileTime gives us the value relative to the NT epoch (Jan
1 1601) rather than the Unix epoch (Jan 1 1970).  However, this function
has a low resolution (~10ms).  Newer versions of Windows provide
GetSystemTimePreciseAsFileTime which gives us a much more accurate time
(<1us).  Unfortunately, the latter is only available on Windows 8+ when
targeting desktop apps.

llvm-svn: 290803