Give conversions of block pointers to ObjC pointers a different cast kind

Give conversions of block pointers to ObjC pointers a different cast kind
than conversions of C pointers to ObjC pointers.  In order to ensure that
we've caught every case, add asserts to CastExpr that strictly determine
which cast kind is used for which kind of bit cast.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139352 91177308-0d34-0410-b5e6-96231b3b80d8