[libc++][doc] Use issue labels.

During the review of D111166 I had a private discussion with @ldionne to
avoid the duplication of the C++2b issues in the Ranges and Format
status pages. The main reason for duplicating them is to make it easier to
find them. The title of the paper may not always make it clear to which
project the paper belongs.

This commit removes all LWG-issues from the Ranges and Format status page
and adds labels for these issue in the C++20/C++23 issues list.

A quick scan revealed there are some issues that are missing a label since
they weren't on the ranges issue list. These can be labelled in a separate
commit. In that commit I'll also look for issues for the spaceship operator
and chrono.

Reviewed By: Quuxplusone, ldionne, #libc

Differential Revision: https://reviews.llvm.org/D111458

GitOrigin-RevId: cef9978027cf4affde7ae3e4a3389053615bca60
5 files changed