Skip declarations in the case of functions.

This is a revert of r223521 in spirit, if not in content. I am not
sure why declarations ended up in LazilyLinkGlobalValues in the first
place; that will take some more investigation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223763 91177308-0d34-0410-b5e6-96231b3b80d8