[clang] [clang] Implement P3144R2 "Deleting a Pointer to an Incomplete Type..." (PR #97733)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> LGTM, generally speaking, but do we maybe want change the old diagnostic to mention that it’s ‘incompatible w/ C++26’ or however we usually phrase that?
 
Done.

> Oh also, this still needs a release note and an entry in the C++ support docs, doesn’t it?

Done.

I also added `-Wc++2c-compat` flag and corresponding diagnostic group.

https://github.com/llvm/llvm-project/pull/97733