[C2y] Claim conformance to WG14 N3478

This paper made it a constraint violation for a TU to end with an
unterminated multiline comment. This is something Clang has always
diagnosed as an error.

Author: AaronBallman

clang/test/C/C2y/n3478.c

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -verify -std=c2y %s
+// RUN: %clang_cc1 -verify -std=c23 %s
+
+/* WG14 N3478: Yes
+ * Slay Some Earthly Demons XIII
+ *
+ * It was previously UB to end a source file with a partial preprocessing token
+ * or a partial comment. Clang has always diagnosed these.
+ */
+
+// expected-error@+1 {{unterminated /* comment}}
+/*

clang/www/c_status.html

@@ -299,7 +299,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Slay Some Earthly Demons XIII</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3478.pdf">N3478</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="full" align="center">Yes</td>
 	</tr>
     <tr>
       <td>Slay Some Earthly Demons XIV</td>