[mlir][flang] Improve handling of fortran module variables. (#91604)

Currently, only those global variables which are at compile unit scope
are added to the 'globals' list of the DICompileUnit. This does not work
for languages which support modules (e.g. Fortran) where hierarchy
can be
variable -> module -> compile unit.

To fix this, if a variable scope points to a module, we walk one level
up and see if module is in the compile unit scope.

This was initially part of #91582 which adds debug information for
Fortran module variables. @kiranchandramohan pointed out that MLIR
changes should go in separate PRs.