[C2y] Claim conformance to WG14 N3451

This paper allows initialization of anonymous structures and unions via
braced initializers, which is something Clang has supported since
forever.

Author: AaronBallman

clang/test/C/C2y/n3451.c

@@ -0,0 +1,43 @@
+// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic -Wno-gnu-folding-constant %s
+// RUN: %clang_cc1 -verify -std=c23 -Wall -pedantic -Wno-gnu-folding-constant %s
+
+/* WG14 N3451: Yes
+ * Initialization of anonymous structures and unions
+ *
+ * This paper allows initialization of anonymous structure and union members
+ * within the containing object.
+ */
+// expected-no-diagnostics
+
+constexpr struct {
+  int a : 10;
+  int : 12;
+  long b;
+} s = { 1, 2 };
+static_assert(s.a == 1 && s.b == 2);
+
+constexpr union {
+  int : 16;
+  char c;
+} u = {3};
+static_assert(u.c == 3);
+
+constexpr struct {
+  union {
+    float a;
+    int b;
+    void *p;
+  };
+  char c;
+} t = {{.b = 1}, 2};
+static_assert(t.b == 1 && t.c == 2);
+
+constexpr struct {
+  union {
+    float a;
+    int b;
+    void *p;
+  };
+  char c;
+} v = {.b = 1, 2};
+static_assert(v.b == 1 && v.c == 2);

clang/www/c_status.html

@@ -279,7 +279,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Initialization of anonymous structures and unions</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3451.pdf">N3451</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="full" align="center">Yes</td>
 	</tr>
     <tr>
       <td>Slay Some Earthly Demons X</td>