Adjust logic for 'objc_protocol_requires_explicit_implementation' for inherited protocols and protocols already conformed in the class hierarchy.

Per more discussion, 'objc_protocol_requires_explicit_implementation' is
refinement that it mainly adds that requirement that a protocol must be
explicitly satisfied at the moment the first class in the class hierarchy
conforms to it.  Any subclasses that also conform to that protocol,
either directly or via conforming to a protocol that inherits that protocol,
do not need to re-implement that protocol.

Doing this requires first doing a pass on the super class hierarchy,
gathering the set of protocols conformed to by the super classes,
and then culling those out when determining conformance.  This
two-pass algorithm could be generalized for all protocol checking,
and could possibly be a performance win in some cases.  For now
we restrict this change to protocols with this attribute to isolate
the change in logic (especially as the design continues to evolve).

This change needs to be adjusted for properties as well; this
only impacts methods right now.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@202948 91177308-0d34-0410-b5e6-96231b3b80d8