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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > I think we should make these changes to all backported features.
> 
> @zwuis @AaronBallman is it okay to handle that in a separate issue/PR?

Yeah, I think it's fine to handle separately.

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