[clang] Implement -Wcast-qual for C++

Summary:
This way, the behavior of that warning flag
more closely resembles that of GCC.

Do note that there is at least one false-negative (see FIXME in tests).

Fixes PR4802.

Testing:
```
ninja check-clang-sema check-clang-semacxx
```

Reviewers: dblaikie, majnemer, rnk

Reviewed By: dblaikie, rnk

Subscribers: mclow.lists, cfe-commits, alexfh, rnk

Differential Revision: https://reviews.llvm.org/D33102

llvm-svn: 307045