[Sema] Make nullness warnings appear in C++.

Given the following code:

    int *_Nullable ptr;
    int *_Nonnull nn = ptr;

...In C, clang will warn you about `nn = ptr`, because you're assigning
a nonnull pointer to a nullable pointer. In C++, clang issues no such
warning. This patch helps ensure that clang doesn't ever miss an
opportunity to complain about C++ code.

N.B. Though this patch has a differential revision link, the actual
review took place over email.

Differential Revision: http://reviews.llvm.org/D14938

llvm-svn: 255556