Add analysis support for blocks. This includes a few key changes:

- Refactor the MemRegion hierarchy to distinguish between different StackSpaceRegions for locals and parameters.
- VarRegions for "captured" variables now have the BlockDataRegion as their super region (except those passed by reference)
- Add transfer function support to GRExprEngine for BlockDeclRefExprs.

This change also supports analyzing blocks as an analysis entry point
(top-of-the-stack), which required pushing more context-sensitivity
around in the MemRegion hierarchy via the use of LocationContext
objects.  Functionally almost everything is the same, except we track
LocationContexts in a few more areas and StackSpaceRegions now refer
to a StackFrameContext object.  In the future we will need to modify
MemRegionManager to allow multiple StackSpaceRegions in flight at once
(for the analysis of multiple stack frames).

llvm-svn: 90809