[LCG] Just move the allocator (now that we can) when moving a call
graph. This simplifies the custom move constructor operation to one of
walking the graph and updating the 'up' pointers to point to the new
location of the graph. Switch the nodes from a reference to a pointer
for the 'up' edge to facilitate this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206450 91177308-0d34-0410-b5e6-96231b3b80d8