[analyzer] Enforce super-region classes for various memory regions.

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

llvm-svn: 300189