[flang][driver] Fix how output filename is generated

Currently, `flang -c file.f` generates `flang_unparsed_source_file_0.o`.
This is incorrect. This patch:
  * simplifies the logic around output filename generation, and
  * makes sure that `file.o` is produced instead of e.g.
    `flang_unparsed_source_file_0.o` when using the `-c` flag

The output filename generation is moved to a dedicated function. I've
also made a few minor improvements, e.g. marked variables as local,
added comments, refined error messages.

Differential Revision: https://reviews.llvm.org/D105546