[Support/FileSystem] Add sub-second precision for atime/mtime of...

[Support/FileSystem] Add sub-second precision for atime/mtime of sys::fs::file_status on unix platforms

Summary:
getLastAccessedTime() and getLastModificationTime() provided times in nanoseconds but with only 1 second resolution, even when the underlying file system could provide more precise times than that.
These changes add sub-second precision for unix platforms that support improved precision.

Also add some comments to make sure people are aware that the resolution of times can vary across different file systems.

Reviewers: labath, zturner, aaron.ballman, kristina

Reviewed By: aaron.ballman, kristina

Subscribers: lebedev.ri, mgorny, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D54826

llvm-svn: 347530