[ThinLTO] Update ThinLTO cache file atimes when on Windows

ThinLTO cache file access times are used for expiration based pruning
and since Vista, file access times are not updated by Windows by
default:

https://blogs.technet.microsoft.com/filecab/2006/11/07/disabling-last-access-time-in-windows-vista-to-improve-ntfs-performance

This means on Windows, cache files are currently being pruned from
creation time. This change manually updates cache files that are
accessed by ThinLTO, when on Windows.

Patch by Owen Reynolds.

Differential Revision: https://reviews.llvm.org/D47266

llvm-svn: 336276