[flang][driver] Add support for `--version` in the bash wrapper

The bash wrapper script, `flang`, calls `flang-new -fc1` under the hood,
which does not support `--version` (this is consistent with `clang -cc1
--version`). This change is needed for `flang --version` to work as
expected.

Note that `flang --version` (the Flang bash wrapper script for the
compiler driver) gives rather minimal output compared to `flang-new
--version` (the Flang compiler driver). As the wrapper script is just a
temporary solution for us, this should be sufficient.

Differential Revision: https://reviews.llvm.org/D105352