Change Sema::PropertyIfSetterOrGetter to make use of isPropertyAccessor.

Old algorithm:
1. See if the name looks like a getter or setter.
2. Use the name to look up a property in the current ObjCContainer
   and all its protocols.
3. If the current container is an interface, also look in all categories
   and superclasses (and superclass categories, and so on).

New algorithm:
1. See if the method is marked as a property accessor. If so, look through
   all properties in the current container and find one that has a matching
   selector.
2. Find all overrides of the method using ObjCMethodDecl's
   getOverriddenMethods. This collects methods in superclasses and protocols
   (as well as superclass categories, which isn't really necessary), and
   checks if THEY are accessors. This part is not done recursively, since
   getOverriddenMethods is already recursive.

This lets us handle getters and setters that do not match the property
names.

llvm-svn: 165627