Revert D135427 "[LTO] Make local linkage GlobalValue in non-prevailing COMDAT available_externally"

This reverts commit 8ef3fd8d.

I mentioned that GlobalAlias was not handled. It turns out GlobalAlias has to be handled in the same patch (as opposed to in a follow-up),
as otherwise clang codegen of C5/D5 constructor/destructor would regress (https://reviews.llvm.org/D135427#3869003).