[clang] Use `cwg_index.html` from GitHub for DR status page (#90352)

Currently we're using official publication of CWG issue list available
at https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_index.html.
Unfortunately, it's not updated as frequently as we sometimes need. For
instance, recently there was a confusion during review of CWG2149 test
(https://github.com/llvm/llvm-project/pull/90079#discussion_r1580174003).
This patch changes our `make_cxx_dr_status` script to use issue list
from CWG GitHub repository. I confirmed with CWG chair that this is the
most up-to-date source of information on CWG issues.

Changing the source of issue list uncovered previously unhandled
"tentatively ready" status of an issue. This status is considered
unresolved by the script (like `open`, `drafting`, and `review`), as the
resolution might change during face-to-face CWG meeting.

I also noticed that CWG decided to handle 2561 differently from what
we're doing, so this DR is now considered not available in Clang,
despite being declared as available since 18. CC @cor3ntin.

This patch also brings new issues into our DR list, so if someone was
waiting for a newly-filed issue to appear on our status page, it should
appear with this PR.