[analyzer] Traverse the Call Graph in topological order.

Modify the call graph by removing the parentless nodes. Instead all
nodes are children of root to ensure they are all reachable. Remove the
tracking of nodes that are "top level" or global. This information is
not used and can be obtained from the Decls stored inside
CallGraphNodes.

Instead of existing ordering hacks, analyze the functions in topological
order over the Call Graph.

Together with the addition of devirtualizable ObjC message sends and
blocks to the call graph, this gives around 6% performance improvement
on several large ObjC benchmarks.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170826 91177308-0d34-0410-b5e6-96231b3b80d8

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