[libc++] Mark some recent LWG issues and papers as done (#87502)

Justifications:
- LWG3950: Done in #66206
- LWG3975: Wording changes only
- LWG4011: Wording changes only
- LWG4030: Wording changes only
- LWG4043: Wording changes only
- LWG3036 and P2875R4: We implemented neither, but the latter reverts
the former, so now we implement both without doing anything!