[clang] Add sections to C++ DR status page (#165749)

Following [recent
discussion](http://lists.isocpp.org/core/2025/10/18637.php) on CWG
reflector, `cwg_index.html` now contains stable name of the section for
each Core issue. I thought that this would be a useful information to
have on our C++ DR status page for that handful of experts who actually
open it, so here we are.

This PR consists of 3 parts:
1. Rewrite of a small routine that parses `cwg_index.html` from splits
and indicies to a single regular expression with named groups, adding
section information.
2. Changes to the rest of the `make_cxx_dr_status` to accommodate for
the first part.
3. Regenerated `cxx_dr_status.html`.

Ideally this PR would only add lines to `cxx_dr_status.html`, but
previously we've been leaving some newlines in issue titles (which never
affected how this page is rendered), which are now properly replaced
with whitespaces — hence a couple of deletions in that file.

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Co-authored-by: Richard Smith <[email protected]>

Author: Endilll