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.

llvm-svn: 135743