[PATCH] DebugInfo: fix crash with null streamer

[Rafael Espíndola]

> With a sufficient change in abstractions it might be possible, though.
>
> Here's the issue (vaguely) as I dabbled with this recently: DWARF line
> table construction is built into the MCStreamer - my understanding is
> that it needs to be at that layer since that's where relaxation
> occurs, so the streamer's the only thing that knows literally how many
> bytes the instruction sequence is (or, in the case of asm output, #
> line directives have to be interspersed with the instruction stream).
> I suppose that's the critical thing to your proposal here: to create
> the line table, information from the MCStreamer is required.
>
> This would be fine for your proposal, but for the wrinkle that the
> rest of DWARF output references the line table too - the files listed
> in the line table are also referenced from debug info to describe the
> file location of the various debug info entities (classes, functions,
> variables, etc). So we need to be able to query for existing file
> numberings, and/or insert new ones (which might later be reused by the
> line table data too).
>
> That's my rough understanding, anyway.

I think in the end the real fix is to fix pr18716. With that fixed the
logic for creating ids can go out of the streamer.

Cheers,
Rafael