Only suppress instance context if a member is actually
accessible in its declaring class;  otherwise we might
fail to apply [class.protected] when considering
accessibility in derived classes.

Noticed by inspection; <rdar://13270329>.

I had an existing test wrong.  Here's why it's wrong:

Follow the rules (and notation) of [class.access]p5.
The naming class (N) is B and the context (R) is D::getX.
- 'x' as a member of B is protected, but R does not occur
  in a member or friend of a class derived from B.
- There does exist a base class of B, A, which is accessible
  from R, and 'x' is accessible at R when named in A because
  'x' as a member of A is protected and R occurs in a member
  of a class, D, that is derived from A;  however, by
  [class.protected], the class of the object expression must
  be equal to or derived from that class, and A does not
  derive from D.