Revert "[flang] Refine output file generation"

This reverts commit fd21d1e1.

The test added in this patch [1] is failing on Windows and causing the
Windows BuildBot [2] to fail. I don't see any obvious way to fix this,
so reverting in order to investigate.

[1] llvm-project/flang/test/Driver/output-paths.f90
[2] https://lab.llvm.org/buildbot/#/builders/172/builds/2077