Fix a horrible bug in our handling of C-style casting, where a C-style

derived-to-base cast that also casts away constness (one of the cases
for static_cast followed by const_cast) would be treated as a bit-cast
rather than a derived-to-base class, causing miscompiles and
heartburn.

Fixes <rdar://problem/8913298>.

llvm-svn: 124340