[Flang][Debug] Modifications for getting pathname

-> Use file pathname from the Flang frontend. It is the frontend
that is in-charge of finding the files and is hence the canonical
source for paths.
-> Convert pathname to absolute pathname while creating the moduleOp.

Co-authored-by: Peter Klausler <pklausler@nvidia.com>

Reviewed By: PeteSteinfeld, vzakhari, jeanPerier, awarzynski

Differential Revision: https://reviews.llvm.org/D141674