Don't bother dynamic loading the Windows Slim RW Lock API.

This API has been present since XP, and I think it's safe to
drop support for XP (since other things have been introduced long
ago which already don't work on XP anyway).

With this patch, we can statically bind against the exports and
not bother falling back to a CRITICAL_SECTION if we can't load
the API.

llvm-svn: 237402