Revert "Restore with fix "[ThinLTO] Ensure we always select the same function copy to import""

This reverts commit r308114 (and follow on fixes to test).

There is a linking failure in a ThinLTO bot:
http://green.lab.llvm.org/green/job/clang-stage2-configure-Rthinlto_build/3663/

(and undefined reference). It seems like it must be a second order
effect of the heuristic change I made, and may take some time to try
to reproduce locally and track down. Therefore, reverting for now.

llvm-svn: 308206