[clang] [C++26] [Contracts] Support Parser, AST and AST Serialization (1/n) (PR #205739)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

Are you aware of https://github.com/efcs/llvm-project/tree/contracts-nightly ?

I'd rather we work on upstreaming that - which has been already reviewed by a couple of people and extensively tested than starting from scratch.

(Eric is no longer working on contracts)

https://github.com/llvm/llvm-project/pull/205739