Reduce -Wuninitialized time by 22% (on sqlite) by removing the recursive AST crawl.

This is accomplished by forcing the needed expressions for -Wuninitialized to always be CFGElements in the CFG.
This allows us to remove a fair amount of the code for -Wuninitialized.

Some fallout:
- AnalysisBasedWarnings.cpp now specifically toggles the CFGBuilder to create a CFG that is suitable for -Wuninitialized.  This
is a layering violation, since the logic for -Wuninitialized is in libAnalysis.  This can be fixed with the proper refactoring.
- Some of the source locations for -Wunreachable-code warnings have shifted.  While not ideal, this is okay because that analysis
already needs some serious reworking.

llvm-svn: 135480