GlobalOpt: Don't drop unused memberes of a Comdat

A linkonce_odr member of a COMDAT shouldn't be dropped if we need to
keep the entire COMDAT group.

This fixes PR21191.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219283 91177308-0d34-0410-b5e6-96231b3b80d8