ObjC kindof: check the context when inserting methods to global pool.

To make kindof lookup work, we need to insert methods with different
context into the global pool, even though they have the same siganture.

Since diagnosis of availability is performed on the best candidate,
which is often the first candidate from the global pool, we prioritize
the methods that are unavaible or deprecated to the head of the list.

Since we now have more methods in the global pool, we need to watch
out for performance impact.

rdar://25635831

llvm-svn: 265877