tip-of-trunk clang has corrected some access checks for special members in a virtual inheritance hierarchy.  Change a few private inheritances to protected.  This change will not impact what the test was testing.  This fixes http://llvm.org/bugs/show_bug.cgi?id=16753.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@187429 91177308-0d34-0410-b5e6-96231b3b80d8