[flang-commits] [flang] [flang] Support -gline-directives-only (PR #217132)

[Abid Qadeer via flang-commits]

abidh wrote:

@VeeEM Heads up, this turned out to expose a backend crash once DWARF 5 is in play. A unit with `emissionKind: DebugDirectivesOnly` still asks for a name table, and the DWARF 5 accelerator table then refers to a compile
unit header that is never emitted:

```console
$ flang -gdwarf-5 -gline-directives-only -S -o /dev/null test.f90
Assertion `LabelBegin && "LabelBegin is not initialized"' failed.
```

Nothing wrong with this patch as such; flang simply never produced such a unit before, so the combination was unreachable. I have opened #218402 to leave the name table off for that emission kind, which is what clang does.

https://github.com/llvm/llvm-project/pull/217132