[Flang] Fix path strings in CreateFIRLangRef.py

In D129186, CreateFIRLangRef.py was created to help fix
formatting problems with FIRLangRef.md.

However, due to an error while rebasing, the last edit was lost,
as such the current path variables are Unix-style.

This patch addresses what was discussed in the previous patch and
now uses os.path.join instead.

Differential Revision: https://reviews.llvm.org/D129644