[clang] [llvm] [Clang] Add __builtin_assume_dereferenceable to encode deref assumption. (PR #121789)

[Erich Keane via llvm-commits]

https://github.com/erichkeane approved this pull request.

1 more question on the semantics + a typo, but the code LGTM.

https://github.com/llvm/llvm-project/pull/121789