Implement final piece of DR963 and also DR587:

Implement final piece of DR963 and also DR587:

A conditional operator between glvalues of types cv1 T and cv2 T produces a
glvalue if the expressions are of the same value kind and one of cv1 and cv2
is a subset of the other.

A conditional operator between two null pointer constants is permitted if one
of them is of type std::nullptr_t.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161476 91177308-0d34-0410-b5e6-96231b3b80d8