Restore "Support for ThinLTO function importing and symbol linking."

This restores commit r251837, with the new library dependence added to
llvm-link/Makefile to address bot failures.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251866 91177308-0d34-0410-b5e6-96231b3b80d8
14 files changed