[lit] Harmonize test timing data between Unix and Windows

The "path" recorded for timing purposes is only used as a key into a dictionary. It is never used as an actual path to a filesystem API, therefore we should use '/' as the canonical separator so that Unix and Windows machines can share timing data. This also ensures that the lit testing works across platforms.

Reviewed By: jhenderson, jmorse

Differential Revision: https://reviews.llvm.org/D98767