[libc++] [NFC] Audit LWG issues for "Nothing To Do" and "Complete".

Each "Nothing To Do" issue only changed nits in the English wording,
not anything to do with the code.
Each "Complete" issue was completed already, as far as I can tell.
I tried to err on the side of caution: I didn't mark a few issues
whose P/Rs were very invasive and would take time to verify, and I
didn't mark a lot of issues involving features we haven't even started
yet.

Differential Revision: https://reviews.llvm.org/D117960