[NFC] Make it more convenient to create canonical PackTypes

Author: rjmccall

include/swift/AST/Types.h

@@ -6740,6 +6740,8 @@ class PackType final : public TypeBase, public llvm::FoldingSetNode,
   }
 };
 BEGIN_CAN_TYPE_WRAPPER(PackType, Type)
+  static CanPackType get(const ASTContext &ctx, ArrayRef<CanType> elements);
+
   CanType getElementType(unsigned elementNo) const {
     return CanType(getPointer()->getElementType(elementNo));
   }

lib/AST/ASTContext.cpp

@@ -3302,6 +3302,11 @@ PackType *PackType::getEmpty(const ASTContext &C) {
   return cast<PackType>(CanType(C.TheEmptyPackType));
 }
 
+CanPackType CanPackType::get(const ASTContext &C, ArrayRef<CanType> elements) {
+  SmallVector<Type, 8> ncElements(elements.begin(), elements.end());
+  return CanPackType(PackType::get(C, ncElements));
+}
+
 PackType *PackType::get(const ASTContext &C, ArrayRef<Type> elements) {
   RecursiveTypeProperties properties;
   bool isCanonical = true;