Drop comdats from the dst module if they are not selected.

Drop comdats from the dst module if they are not selected.

A really unfortunate design of llvm-link and related libraries is that
they operate one module at a time.

This means they can copy a GV to the destination module that should not
be there in the final result because a later bitcode file takes
precedence.

We already handled cases like a strong GV replacing a weak for example.

One case that is not currently handled is a comdat replacing another.
This doesn't happen in ELF, but with COFF largest selection kind it is
possible.

In "llvm-link a.ll b.ll" if the selected comdat was from a.ll,
everything will work and we will not copy the comdat from b.ll.

But if we run "llvm-link b.ll a.ll", we fail to delete the already
copied comdat from b.ll. This patch fixes that.

llvm-svn: 264103