When we walk a ClassDecl, generate its vtable, first pulling in decls from its ancestor classes, then overlaying overridden or new decls as we discover them.
Swift SVN r8947
These will provide a SIL-level representation of class_method dispatch, mapping from dynamically-dispatched SILDeclRefs to SILFunctions so that devirtualization passes will be able to promote a class_method for a statically-known type to a function_ref without going all the way back to the AST.
Swift SVN r8943
