[clang-tools-extra] [clang-tidy] Support C++26 placeholder bindings (PR #207604)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

> ... Just to make sure I'm reading this right: is this a longer-term direction you wanted to flag, or something you'd want resolved before this PR can merge? Happy to go either way.

It's a long-term direction to me but other reviewers might have different thought.

https://github.com/llvm/llvm-project/pull/207604