r211844 - Add test coverage for .bc input in the frontend

[Rafael Espíndola]

> Any idea why ELF assembly emits this special initial '.file' directive
> that's derived from the active tool's input filename rather than IR, as with
> subsequent '.file' directives? (Or, indeed why the other asm formats don't
> share the behaviour?)

It is confusing. There are two .file directives in ELF:

.file foo  -> this produces a FILE symbol with that name.
.file num foo -> this creates a debug info file with number num.

I agree that the ".file foo" directive should not use the IL name and
should instead be encoded in the IL, like we do for .ident.

Cheers,
Rafael