[NFC] Add a function to induce a pack type from a slice of CanParams.

Author: rjmccall

include/swift/AST/Types.h

@@ -6866,6 +6866,8 @@ class PackType final : public TypeBase, public llvm::FoldingSetNode,
 BEGIN_CAN_TYPE_WRAPPER(PackType, Type)
   static CanPackType get(const ASTContext &ctx, ArrayRef<CanType> elements);
   static CanPackType get(const ASTContext &ctx, CanTupleEltTypeArrayRef elts);
+  static CanPackType get(const ASTContext &ctx,
+                         AnyFunctionType::CanParamArrayRef params);
 
   static CanTypeWrapper<PackType>
   getSingletonPackExpansion(CanType packParameter);

lib/AST/ASTContext.cpp

@@ -3305,6 +3305,16 @@ CanPackType CanPackType::get(const ASTContext &C,
   return CanPackType(PackType::get(C, ncElements));
 }
 
+CanPackType CanPackType::get(const ASTContext &C,
+                             AnyFunctionType::CanParamArrayRef params) {
+  SmallVector<Type, 8> ncElements;
+  ncElements.reserve(params.size());
+  for (auto param : params) {
+    ncElements.push_back(param.getParameterType());
+  }
+  return CanPackType(PackType::get(C, ncElements));
+}
+
 PackType *PackType::get(const ASTContext &C, ArrayRef<Type> elements) {
   RecursiveTypeProperties properties;
   bool isCanonical = true;