[flang][driver] Extend the `flang` bash script to act as a driver

Until now, `f18` would:
  1. Use Flang to unparse the input files
  2. Call an external Fortran compiler to compile the unparsed source
  files (generated in step 1)

With this patch, `f18` will stop after unparsing the input source files,
i.e. step 1 above. The `flang` bash script will take care of step 2,
i.e. calling an external Fortran compiler driver to compile them. This
way:
  * the functionality of `f18` is reduced - it will only drive Flang (as
  opposed to delegating code-generation to an external tool on top of
  this)
  * we will able to switch between `f18` and `flang-new` for unparsing before
  an external Fortran compiler is called for code-generation

The updated `flang` bash script needs to specify the output file when
using the `-fdebug-unparse` action. Both `f18` and `flang-new` have been
updated accordingly.

These changes were discussed in [1] as a requirement for replacing `f18`
with `flang-new`.

[1] https://lists.llvm.org/pipermail/flang-dev/2021-April/000677.html

Differential Revision: https://reviews.llvm.org/D103177

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