[clang] [RFC] Initial implementation of P2719 (PR #113510)

[via cfe-commits]

cor3ntin wrote:

I'm getting nitpicky, but there are still unaddressed comments.

In the interest of landing that soon, we should figure out the following:

Do we want to keep the document, knowing this is likely to be adopted as a standard feature? I would rather just link to the paper.
- We need a changelog entry
- Do we want to keep the compiler flags knowing this is likely to be adopted as a language feature? I would prefer checking for c++26 + extension warnings in older language modes
- Do we actually want to set the feature test macro now?
- Why do we have both a feature test macro and `has_cxx_feature` ?

I do think landing the PR ahead of Sofia makes perfect sense. This is a large body of work that Apple is keen on seeing upstreamed. It's a great security feature, and the paper is past EWG with strong support.

The next standard meetings will be well ahead of the clang 21 feature freeze, so we can reassess then if WG21... surprises us.

@erichkeane @AaronBallman 

https://github.com/llvm/llvm-project/pull/113510