[ADT] Simplify determining size for joining items

[jloser]

Summary

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

llvm/include/llvm/ADT/StringExtras.h

 
 inline size_t join_one_item_size(char) { return 1; }
 inline size_t join_one_item_size(const char *S) { return S ? ::strlen(S) : 0; }
 
 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
 
 /// Joins the strings in the range [Begin, End), adding Separator between
 /// the elements.
 template <typename IteratorT>
 inline std::string join(IteratorT Begin, IteratorT End, StringRef Separator) {