Fix massive LiveVariables regression (due to LiveVariables rewrite) by addressing two performance problems:

- Speed of "merge()", which merged data flow facts.  This was doing a set canonicalization on every insertion, which was super slow.
  To fix this, we use ImmutableSetRef.

- Visit CFGBlocks in reverse postorder.  This is a huge speedup, as on some test cases the algorithm would take many iterations
  to converge.

This contains a bunch of copy-paste from UninitializedValues.cpp and ThreadSafety.cpp.  The idea
was to get something working first, and then refactor the common logic for all three files into
a separate analysis/library entry point.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139968 91177308-0d34-0410-b5e6-96231b3b80d8

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