[C99] Claim full support for WG14 N772

No tests are added because existing tests cover this. The thrust of the
paper is that an implementation needs to support including a header
file whose name is eight character or less and contains only letters
and numbers; we definitely manage this.

Author: AaronBallman

clang/www/c_status.html

@@ -369,7 +369,7 @@ <h2 id="c99">C99 implementation status</h2>
     <tr>
       <td>relaxed restrictions on portable header names</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n772.htm">N772</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="full" align="center">Yes</td>
     </tr>
     <tr>
       <td>return without an expression not permitted in function that returns a value</td>