Implement DR7

Disallowing deriving from classes that have private virtual base classes
except in instances where the deriving class would be able to cast
itself to the private virtual base via a different derivation.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@183462 91177308-0d34-0410-b5e6-96231b3b80d8