[clang] [clang][NFC] Mark CWG2943 as implemented and add a test (PR #195192)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

We need additional test cases:
1) calling via reference to function (nodiscard shouldn't work on them)
2) calling via pointer to function (nodiscard shouldn't work on them either)
3) calling via splice-expression (we don't implement them, so put it behind `#if __cpp_impl_reflection`) (nodiscard should work)

https://github.com/llvm/llvm-project/pull/195192