[clang] [C2y] Add test coverage and documentation for WG14 N3341 (PR #115478)

[Mariya Podchishchaeva via cfe-commits]

================
@@ -191,7 +191,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Slay Some Earthly Demons III</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3341.pdf">N3341</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="full" align="center">Yes</td>
----------------
Fznamznon wrote:

I wonder why the patch comes with release note, yet the official support is not claimed starting from clang 20?

https://github.com/llvm/llvm-project/pull/115478