GlobalOpt is already dead-code-eliminating global definitions. With
this change it also takes care of declarations.
Hopefully this should make it now a strict superset of GlobalDCE.
This is important for LTO/ThinLTO as we don't want the linker to see
"undefined reference" when it processes the input files: it could
prevent proper internalization (or even load an extra file from a
static archive, changing the behavior of the program!).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281653 91177308-0d34-0410-b5e6-96231b3b80d8
