[C2y] Claim nonconformance to WG14 N3410

This paper made it a constraint violation for the same identifier
within a TU to have both internal and external linkage. It was
previously UB.

Clang does not correctly diagnose the constraint in some cases,
documented in the added test case.

Author: AaronBallman

clang/test/C/C2y/n3410.c

@@ -0,0 +1,45 @@
+// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic -Wno-unused %s
+
+/* WG14 N3410: No
+ * Slay Some Earthly Demons XI
+ *
+ * It is now ill-formed for the same identifier within a TU to have both
+ * internal and external linkage.
+ */
+
+void func1() {
+  extern int a; // #a
+}
+
+// This 'a' is the same as the one declared extern above.
+static int a; /* expected-error {{static declaration of 'a' follows non-static declaration}}
+                 expected-note@#a {{previous declaration is here}}
+               */
+
+static int b;
+void func2() {
+  // This 'b' is the same as the one declaraed static above, but this is not
+  // ill-formed because of C2y 6.2.2p4, which gives this variable internal
+  // linkage because the previous declaration had internal linkage.
+  extern int b; // Ok
+}
+
+static int c, d;
+void func3() {
+  int c; // no linkage, different object from the one declared above.
+  for (int d;;) {
+    // This 'c' is the same as the one declared at file scope, but because of
+    // the local scope 'c', the file scope 'c' is not visible.
+    // FIXME: This should be diagnosed under N3410.
+    extern int c;
+    // This 'd' is the same as the one declared at file scope as well, but
+    // because of the 'd' declared within the for loop, the file scope 'd' is
+    // also not visible, same as with 'c'.
+    // FIXME: This should be diagnosed under N3410.
+    extern int d;
+  }
+  for (static int e;;) {
+    extern int e; // Ok for the same reason as 'b' above.
+  }
+}
+

clang/www/c_status.html

@@ -289,7 +289,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Slay Some Earthly Demons XI</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3410.pdf">N3410</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="none" align="center">No</td>
 	</tr>
     <tr>
       <td>Slay Some Earthly Demons XII</td>