Fix build failure with MSVC in C++20 mode

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

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D116904