Update thin-lto cache file atimes when on windows

Hi all,

After experimenting with thin-lto caching behaviour I found an issue with pruning when on Windows.

Thin-lto cache file atimes are used for expiration based pruning, and since Vista, access file 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.

I’ve made a patch for this so any feedback would be appreciated:

D47266

Thanks,

Owen

+pcc for LTO caching and +inglorion for Windows specific issues.
Teresa