[flang][driver] Fix output filename generation in `flang`

In the `flang` bash script, we need to be careful _when_ to use <output>
from `flang -c -o <output> <input>` when generating the relocatable
output file name.

In particular, we should use it in this case:
```compilation only
flang -c -o <output> <input>
```
but leave it for the final executable here:
```compile, assemble and link
flang  -o <output> <input>
```
This change is implemented in `get_relocatable_name`.

I've also taken the liberty to fix how errors from sub-commands are
reported (without this change, `flang` always returns `0` on failure).
This is implemented in `main`.

Differential Revision: https://reviews.llvm.org/D105896