Fix a crash in Retain Count checker error reporting

Fixes a crash in Retain Count checker error reporting logic by handing
the allocation statement retrieval from a BlockEdge program point.

Also added a simple CFG dump routine for debugging.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210960 91177308-0d34-0410-b5e6-96231b3b80d8