[clang] [CodeGen] Apply 'readonly' to 'this' pointer argument. (PR #106499)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 approved this pull request.

Sounds good to me and I add another few reviewers to take another look.

https://github.com/llvm/llvm-project/pull/106499