[clang] [Clang][C++23] Implement P1774R8: Portable assumptions (PR #81014)

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cor3ntin wrote:

Thanks for working on that.

GCC does checks assumption in constant evaluation. I think we should too.

My main question is whether we think supporting `assume` in the front-end is going to be a net positive knowing that the backend is going to pessimize some use of the attribute 

https://discourse.llvm.org/t/llvm-assume-blocks-optimization/71609

We have 2 options:
 - Wait for llvm to get improve their handling of assumption based optimizations.
 - Proceed with this work hoping this encourages work on the optimizer.

I don't have a strong opinion, but we should collectively be aware of pitfalls and
https://discourse.llvm.org/t/llvm-assume-blocks-optimization/71609  









https://github.com/llvm/llvm-project/pull/81014