[clang] [NFC] [C++] [Modules] Mark P2615 as implemented and add test (PR #147135)

[Chuanqi Xu via cfe-commits]

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

LGTM.  Thanks. A small suggestion is, it is better to paste the link of the paper. So that I don't have to find it my self.

https://github.com/llvm/llvm-project/pull/147135