In Objective-C, pull arbitrary attributes from overridden

In Objective-C, pull arbitrary attributes from overridden
methods, including indirectly overridden methods like those
declared in protocols and categories.  There are mismatches
that we would like to diagnose but aren't yet, but this
is fine for now.

I looked at approaches that avoided doing this lookup
unless we needed it, but the infer-related-result-type
checks were doing it anyway, so I left it with the same
fast-path check for no previous declartions of that
selector.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@135743 91177308-0d34-0410-b5e6-96231b3b80d8