[STLExtras] Undo C++20 hack

c312f02 worked around a bug here, but
that has since been fixed in 47747da

Author: d0k

llvm/include/llvm/ADT/STLExtras.h

@@ -1291,15 +1291,11 @@ class indexed_accessor_range_base {
   }
 
   /// Compare this range with another.
-  template <typename OtherT>
-  friend bool operator==(const indexed_accessor_range_base &lhs,
-                         const OtherT &rhs) {
-    return std::equal(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
-  }
-  template <typename OtherT>
-  friend bool operator!=(const indexed_accessor_range_base &lhs,
-                         const OtherT &rhs) {
-    return !(lhs == rhs);
+  template <typename OtherT> bool operator==(const OtherT &rhs) const {
+    return std::equal(begin(), end(), rhs.begin(), rhs.end());
+  }
+  template <typename OtherT> bool operator!=(const OtherT &rhs) const {
+    return !(*this == rhs);
   }
 
   /// Return the size of this range.