Fix several problems with protected access control:

  - The [class.protected] restriction is non-trivial for any instance
    member, even if the access lacks an object (for example, if it's
    a pointer-to-member constant).  In this case, it is equivalent to
    requiring the naming class to equal the context class.
  - The [class.protected] restriction applies to accesses to constructors
    and destructors.  A protected constructor or destructor can only be
    used to create or destroy a base subobject, as a direct result.
  - Several places were dropping or misapplying object information.

The standard could really be much clearer about what the object type is
supposed to be in some of these accesses.  Usually it's easy enough to
find a reasonable answer, but still, the standard makes a very confident
statement about accesses to instance members only being possible in
either pointer-to-member literals or member access expressions, which
just completely ignores concepts like constructor and destructor
calls, using declarations, unevaluated field references, etc.

llvm-svn: 154248