[STLExtras] Remove incorrect hack to make indexed_accessor_range operator== compatible with C++20 (#72220)

This partially reverts c312f02

The motivation behind this is unclear and the change predates the clang
[implementation](38b9d31)
of
[p2468r2](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2468r2.html)
so I am not sure if it was ever intended to work. Rewritten template
operators were broken since the beginning.

Moreover, moving away from `friend` would be beneficial as these would
only accepted once clang revises its implementation for fixing
#70210. It also helps in
making sure that older compilers still compile LLVM (in C++20).

Author: usx95

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.