[clang][DR] Test and mark DR1305 as complete

[zero9178]

Summary

DR: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1305

This DR has always been implemented in all clang versions as far back as I could go. This patch basically just adds a test with the needed comments to mark it as complete.

[cor3ntin]

Looks good except I think it makes more sense to put the new test just before the first test

clang/test/CXX/drs/dr13xx.cpp

15–25

I think it would make more sense to put the new test here

[erichkeane]

LGTM!

[zero9178]

Address reviewers comments: Group test nicely with the other dr namespaces.

[cor3ntin]

Looks great!

clang/test/CXX/drs/dr13xx.cpp

 // RUN: %clang_cc1 -std=c++98 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++17 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 
 __extension__ typedef __SIZE_TYPE__ size_t;
 
 namespace std {
   template<typename T> struct initializer_list {
     const T *ptr;
     size_t n;
     initializer_list(const T*, size_t);
   };
 }
 
+#if __cplusplus >= 201103L
+namespace dr1305 { // dr1305: yes
+struct Incomplete; // expected-note {{forward declaration of 'dr1305::Incomplete'}}
+struct Complete {};
+
+int incomplete = alignof(Incomplete(&)[]); // expected-error {{invalid application of 'alignof' to an incomplete type 'dr1305::Incomplete'}}
+int complete = alignof(Complete(&)[]);
+}
+#endif
+
 namespace dr1310 { // dr1310: 5
   struct S {} * sp = new S::S; // expected-error {{qualified reference to 'S' is a constructor name}}
   void f() {
     S::S(a); // expected-error {{qualified reference to 'S' is a constructor name}}
   }
   struct T { int n; typedef int U; typedef T V; };
   int k = T().T::T::n;
   T::V v;

clang/www/cxx_dr_status.html

<tr class="open" id="1304">
     <td>drafting</td>
     <td>Omitted array bound with string initialization</td>
     <td align="center">Not resolved</td>
   </tr>
   <tr id="1305">
     <td><a href="https://wg21.link/cwg1305">1305</a></td>
     <td>CD3</td>
     <td><TT>alignof</TT> applied to array of unknown size</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="1306">
     <td><a href="https://wg21.link/cwg1306">1306</a></td>
     <td>CD3</td>
     <td>Modifying an object within a <TT>const</TT> member function</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1307">