Re-land "Add LazyCallGraph API to add function to RefSCC"

This re-commits https://reviews.llvm.org/D70927, which I reverted in
https://reviews.llvm.org/rG28213680b2a7d1fdeea16aa3f3a368879472c72a due
to a buildbot error:
http://lab.llvm.org:8011/builders/clang-cmake-x86_64-avx2-linux/builds/13251

I no longer include a test case that appears to crash when built with the
buildbot's compiler, GCC 5.4.0.