[PrintObjC] Fix ReferencedTypeFinder::isConstrained to filter out invertible protocols

The checking should be as simple as checking for `isAny()` or `isAnyObject()`
on an existential type because all of the non-inverse protocol requirements
are going to form a protocol composition type.

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -78,10 +78,8 @@ class ReferencedTypeFinder : public TypeDeclFinder {
   /// class-bound ("conforms to" AnyObject).
   static bool isConstrained(GenericSignature sig,
                             GenericTypeParamType *paramTy) {
-    if (sig->getSuperclassBound(paramTy))
-      return true;
-
-    return !sig->getRequiredProtocols(paramTy).empty();
+    auto existentialTy = sig->getExistentialType(paramTy);
+    return !(existentialTy->isAny() || existentialTy->isAnyObject());
   }
 
   Action visitBoundGenericType(BoundGenericType *boundGeneric) override {