Fix build failure with MSVC in C++20 mode

[Godin]

Summary

Without this patch when using CMAKE_CXX_STANDARD=20
and MSVC 19.30.30705.0 compilation fails with

clang\lib\Tooling\Syntax\Tree.cpp(347): error C2666: 'clang::syntax::Tree::ChildIteratorBase<clang::syntax::Tree::ChildIterator,clang::syntax::Node>::operator ==': 4 overloads have similar conversions
clang\lib\Tooling\Syntax\Tree.cpp(392): error C2666: 'clang::syntax::Tree::ChildIteratorBase<clang::syntax::Tree::ChildIterator,clang::syntax::Node>::operator ==': 4 overloads have similar conversions

Note that removed comment that
"iterator_facade_base requires == to be a member"
was made obsolete by change https://reviews.llvm.org/D78938

[sammccall]

Thanks! Much nicer :-)

[Godin]

Thank you for the review @sammccall !
Can someone please commit this on my behalf since I do not have commit rights?

clang/include/clang/Tooling/Syntax/Tree.h

 //===- Tree.h - structure of the syntax tree ------------------*- C++ -*-=====//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // Defines the basic structure of the syntax tree. There are two kinds of nodes:
class Tree : public Node {
   protected:
     NodeT *N = nullptr;
     using Base = ChildIteratorBase;
 
   public:
     ChildIteratorBase() = default;
     explicit ChildIteratorBase(NodeT *N) : N(N) {}
 
-    bool operator==(const DerivedT &O) const { return O.N == N; }
+    friend bool operator==(const DerivedT &LHS, const DerivedT &RHS) {
+      return LHS.N == RHS.N;
+    }
+
     NodeT &operator*() const { return *N; }
     DerivedT &operator++() {
       N = N->getNextSibling();
       return *static_cast<DerivedT *>(this);
     }
 
     /// Truthy if valid (not past-the-end).
     /// This allows: if (auto It = find_if(N.children(), ...) )
private:
   /// (!) \p Begin can be null to model an append.
   void replaceChildRangeLowLevel(Node *Begin, Node *End, Node *New);
   friend class MutationsImpl;
 
   Node *FirstChild = nullptr;
   Node *LastChild = nullptr;
 };
 
-// Provide missing non_const == const overload.
-// iterator_facade_base requires == to be a member, but implicit conversions
-// don't work on the LHS of a member operator.
-inline bool operator==(const Tree::ConstChildIterator &A,
-                       const Tree::ConstChildIterator &B) {
-  return A.operator==(B);
-}
-
 /// A list of Elements separated or terminated by a fixed token.
 ///
 /// This type models the following grammar construct:
 /// delimited-list(element, delimiter, termination, canBeEmpty)
 class List : public Tree {
 public:
   template <typename Element> struct ElementAndDelimiter {
     Element *element;