[clang] [clang] Add separate C++23 extension flag for attrs on lambda (PR #74553)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov commented:

This is useful to us to have attributes on lambdas early in C++20 mode before switching to C++23. I only have a nitpick about naming and happy to approve after its addressed.

And let's give @AaronBallman a few days to jump in.

https://github.com/llvm/llvm-project/pull/74553