[ADT] Simplify determining size for joining items

`join_items_size` uses recursive function templates for determining the total
items size.  Instead, use fold expressions since we have C++17 available now.

Differential Revision: https://reviews.llvm.org/D138375

Author: JoeLoser

llvm/include/llvm/ADT/StringExtras.h

@@ -451,14 +451,8 @@ template <typename T> inline size_t join_one_item_size(const T &Str) {
   return Str.size();
 }
 
-inline size_t join_items_size() { return 0; }
-
-template <typename A1> inline size_t join_items_size(const A1 &A) {
-  return join_one_item_size(A);
-}
-template <typename A1, typename... Args>
-inline size_t join_items_size(const A1 &A, Args &&... Items) {
-  return join_one_item_size(A) + join_items_size(std::forward<Args>(Items)...);
+template <typename... Args> inline size_t join_items_size(Args &&...Items) {
+  return (0 + ... + join_one_item_size(std::forward<Args>(Items)));
 }
 
 } // end namespace detail