[clang] Add test for CWG2213

[Endill]

Summary

P1787: CWG2213 is resolved by allowing an elaborated-type-specifier to contain a simple-template-id without friend.
Wording: see changes to [dcl.type.elab]]/1.

The gist of the issue is that forward declaration of partial class template specialization was disallowed.

[Endill]

If reviewers would prefer this to be simply marked N/A, I'm not going to oppose. I wrote a test for this because it was simple enough.

[shafik]

Thank you, I think more examples are never bad, LGTM

clang/test/CXX/drs/dr22xx.cpp

 // RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++1z -triple x86_64-unknown-unknown %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 
 #if __cplusplus >= 201103L
 namespace dr2211 { // dr2211: 8
 void f() {
   int a;
   auto f = [a](int a) { (void)a; }; // expected-error {{a lambda parameter cannot shadow an explicitly captured entity}}
   // expected-note@-1{{variable 'a' is explicitly captured here}}
   auto g = [=](int a) { (void)a; };
 }
 }
 #endif
 
+namespace dr2213 { // dr2213: yes
+template <typename T, typename U>
+struct A;
+
+template <typename U>
+struct A<int, U>;
+} // namespace dr2213
+
 namespace dr2229 { // dr2229: 7
 struct AnonBitfieldQualifiers {
   const unsigned : 1; // expected-error {{anonymous bit-field cannot have qualifiers}}
   volatile unsigned : 1; // expected-error {{anonymous bit-field cannot have qualifiers}}
   const volatile unsigned : 1; // expected-error {{anonymous bit-field cannot have qualifiers}}
 
   unsigned : 1;
   const unsigned i1 : 1;

clang/www/cxx_dr_status.html

<tr id="2212">
     <td>CD5</td>
     <td>Typedef changing linkage after use</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="2213">
     <td><a href="https://cplusplus.github.io/CWG/issues/2213.html">2213</a></td>
     <td>CD6</td>
     <td>Forward declaration of partial specializations</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="2214">
     <td><a href="https://cplusplus.github.io/CWG/issues/2214.html">2214</a></td>
     <td>C++17</td>
     <td>Missing requirement on representation of integer values</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="2215">