[Patch][pr19848] Produce explicit comdats in clang

[Rafael Espíndola]

> So, how about splitting work in:
>
> * llvm: Implement the syntatic change for comdats with the same name
> as the GV, but keep the current representation.
> * clang: this patch, but with less test change noise.
> * llvm: stop producing implicit comdats.
> * llvm: implement and benchmark one of the possible optimizations.

Is everyone OK with this plan? I just started implementing the first patch.

Cheers,
Rafael