[flang] Fix crash with alternate returns in modules

Summary:
We weren't handling the case of subroutines with alternate returns that
are contained in modules.  I changed the code to add an `*` as the name
of the parameter when creating the `.mod` file.

Reviewers: tskeith, klausler, DavidTruby

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82096