Provide option to sync timestamps from source

[kastiglione]

@nvanfleet asked on twitter whether index-import would work when syncing indexes between local checkouts, if they're not on the same git version.

This may work by adjusting the timestamp of output unit files. For example, consider importing MyFile.o-AAAAAAAAAAAA from checkout A into checkout B. If A/MyFile.swift and B/MyFile.swift have different modification times, then this index may be invalid, because A/MyFile.swift is in the future compared to B/MyFile.swift, or vice versa. To resolve this, after importing index-import could update the timestamp of the new unit (MyFile.o-BBBBBBBBBBBB) file to be be earlier than B/MyFile.swift. Xcode would hopefully see the index as being out of date, and re-index B/MyFile.swift. Source files that have matching timestamps would have their timestamp preserved.

[kastiglione]

Or to put it much more succinctly: import only the index files where the source files have the same timestamp.