AST: Simplify TypeBase::getTypeParameterPacks()

Type deletes operator== so this no longer compiles on the next branch.
No idea why it worked before. We could use isEqual() instead but in fact
none of the callers care about uniqueness so let's do the simple thing
that is guaranteed to work.

Author: slavapestov

lib/AST/ParameterPack.cpp

@@ -100,21 +100,16 @@ void TypeBase::walkPackReferences(
 
 void TypeBase::getTypeParameterPacks(
     SmallVectorImpl<Type> &rootParameterPacks) {
-  llvm::SmallSetVector<Type, 2> rootParameterPackSet;
-
   walkPackReferences([&](Type t) {
     if (auto *paramTy = t->getAs<GenericTypeParamType>()) {
       if (paramTy->isParameterPack())
-        rootParameterPackSet.insert(paramTy);
+        rootParameterPacks.push_back(paramTy);
     } else if (auto *archetypeTy = t->getAs<PackArchetypeType>()) {
-      rootParameterPackSet.insert(archetypeTy->getRoot());
+      rootParameterPacks.push_back(archetypeTy->getRoot());
     }
 
     return false;
   });
-
-  rootParameterPacks.append(rootParameterPackSet.begin(),
-                            rootParameterPackSet.end());
 }
 
 bool GenericTypeParamType::isParameterPack() const {