[LCG] Add the most basic of edge insertion to the lazy call graph. This
just handles the pre-DFS case. Also add some test cases for this case to
make sure it works.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207411 91177308-0d34-0410-b5e6-96231b3b80d8