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

[Yanzuo Liu via cfe-commits]

https://github.com/zwuis commented:

Personally I want to name this check `modernize-unused-variables` to support

- adding `[[maybe_unused]]` instead of renaming the variable to `_` (controlled by an option)
- 'simple' variables

in future.

But, I want fixits to add `[[maybe_unused]]` by default, because `[[maybe_unused]]` is introduced earlier and it behaves better with `assert`, so I'd like to keep this name and make it an alias in future. (changing default fixits is a kind of breaking changes to me)

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