[LCG] Switch the Callee sets to be DenseMaps pointing to the index into
the Callee list. This is going to be quite important to prevent removal
from going quadratic. No functionality changed at this point, this is
one of the refactoring patches I've broken out of my initial work toward
mutation updates of the call graph.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206938 91177308-0d34-0410-b5e6-96231b3b80d8