[clang] [Clang] update reasoned delete diagnostic kind to use Extension, making it pedantic only (PR #114713)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM assuming precommit CI comes back green.

Btw, now would probably be a good time for you to get your own commit privileges, you've done a number of quality contributions already: https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access

https://github.com/llvm/llvm-project/pull/114713