Fix equality of TypeWitnessAndDecl

It was incorrectly hashing the type as a pointer
but canonicalizing for equality. Change equality
to compare pointers.

Author: hamishknight

include/swift/AST/Witness.h

@@ -212,7 +212,7 @@ struct TypeWitnessAndDecl {
 
   friend bool operator==(const TypeWitnessAndDecl &lhs,
                          const TypeWitnessAndDecl &rhs) {
-    return lhs.witnessType->isEqual(rhs.witnessType) &&
+    return lhs.witnessType.getPointer() == rhs.witnessType.getPointer() &&
            lhs.witnessDecl == rhs.witnessDecl;
   }