Fix an incredibly nasty iterator invalidation problem. I am too spoiled by ilists :)

Fix an incredibly nasty iterator invalidation problem.  I am too spoiled by ilists :)
Eventually it would be nice if CallGraph maintained an ilist of CallGraphNode's instead
of a vector of pointers to them, but today is not that day.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13100 91177308-0d34-0410-b5e6-96231b3b80d8