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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307045 91177308-0d34-0410-b5e6-96231b3b80d8