[Flang] Add/Restore basic debug support (1/n)

Recent changes to MLIR meant that Flang does not generate any debug line
table information.

This patch adds a pass that provides some foundation work with which
basic line table debug info can be generated. A walk is performed on
all the `func` ops in the module and they are decorated with a fusedLoc
op that contains the debug metadata for the subroutine along with
location information.

Alternatives include populating this info during lowering or during FIR
to LLVM Dialect conversion.

Note: Patches in future will add
    -> more realistic debug info for types and other fields.
    -> driver flags to control generation of debug.

Fixes #58634.

Reviewed By: awarzynski, vzakhari

Differential Revision: https://reviews.llvm.org/D137956