[PATCH] Linker: Replace overridden subprograms

[Rafael Espíndola]

> It's asymmetrical (asymmetry happens all the time in linkers, right?), but
> I'm not sure there's a good way to fix this without invasive changes to
> `MapValue()`.  I'd rather leave "solving this correctly" until we have a
> custom class for subprograms (as mentioned in PR21910).

So the logic is asymmetrical, but the end result is normally
symmetrical, which is why some tests use
; RUN: llvm-link %p/foo.ll %p/Inputs/foo.ll ...
; RUN: llvm-link %p/Inputs/foo.ll %p/foo.ll ...

But yes, this can be done in another patch. Please just leave a FIXME
in the code somewhere.

> (I wouldn't even be trying to solve this yet if it weren't blocking
> our Release+Asserts bootstrap.)

I think the patch is OK with the SrcM->getNamedMetadata("llvm.dbg.cu")
check removed.

Cheers,
Rafael