[rfc][gold plugin] Fix pr19901

[Rafael Espíndola]

> I don't think we drop them yet, but I see it is as just a convenience
> so that it is easy to test it with llc. While developing this patch I
> did a few clang bootstraps and only once noticed a unused linkonce_odr
> in a .bc produced by clang. The previous passes should drop it. We
> could add an option to do it, I am just not sure it would have a big
> impact (I will write a patch to check).

And I can't even reproduce that anymore.

I applied the attached patch to llvm to make it assert when writing
bitcode or assembly with a dead linkonce_odr. It will correctly crash
in llvm-as or llc when given just

define linkonce_odr void @g() {
  ret void
}

But it can successfully build a stage2 with or without LTO, so it
looks we are pretty good at dropping unused linkonce_odr and dropping
it in codegen proper would not add any real value.

Cheers,
Rafael
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