[flang][Driver] The driver does not accept assembly code

When assembly code is passed to the Flang driver, it emits the following message:

```console
$ flang test.s
flang-23: error: flang does not support assembly files as input: 'test.s'
```

Other Fortran compilers (gfortran, ifx) support assembly inputs, so Flang should do as well.

Start by reproducing the reported behavior with the Flang driver command `flang test.s` and trace where assembly-file inputs are rejected. Compare that behavior with the stated gfortran and ifx behavior, then verify that Flang accepts assembly input without emitting the reported error.

Written by the indexing model from the issue text.