We now check the type of the super-region pointer for most SubRegion classes
in compile time; some checks are run-time though.

This is an API-breaking change (we now require explicit casts to specific region
sub-classes), but in practice very few checkers are affected.

Differential Revision: https://reviews.llvm.org/D26838


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300189 91177308-0d34-0410-b5e6-96231b3b80d8