Since commit c76c1ea2 (find_program: Consider CWD only for paths with
separator, 2018-05-31, v3.13.0-rc1~413^2) we accidentally convert the
leading `//` to just `/` on results with Windows network paths.

This was caused by `CollapseCombinedPath` being less robust than
`CollapseFullPath`.  Restore use of the latter but in a way that
preserves the requirement of the above commit to support candidates that
are themselves absolute.

Fixes: #19049
Issue: #19050