[clang] [Clang] Accept auto casts pre-C++23 as an extension (PR #200675)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Can you update the PR summary with more details about why the changes are happening? (Makes it easier on ourselves for code archeology.) This also needs a release note in `clang/docs/ReleseNotes.rst`.

https://github.com/llvm/llvm-project/pull/200675