[clang] Propely handle tests for open DRs in make_cxx_dr_status

A follow-up to D136133. It was mentioned in #58382 that there is a need
to test for DRs that have not been officially resolved yet. This patch
aims to replace original "hackery" with proper handling for such cases.
Highlights:

- Availability can be suffixed (further) with "open", "drafting", or
  "review", e.g. // dr2565: 16 open, // drXXXX: 16 c++17 drafting
- Checks are implemented to ensure that this suffix corresponds to
  actual issue status
- Non-resolved DRs are counted (stdout of make_cxx_dr_status)
- No changes made to cxx_dr_status.html
- 'c++20' availability suffix added
- Remove 'concurrency' status since it's no longer
  on the list of statuses in CWG Active Issues

Differential Revision: https://reviews.llvm.org/D138901