[clang] Add test for CWG1820 "Qualified typedef names" (#91765)

[P1787R6](https://wg21.link/p1787r6):
> [CWG1820](https://cplusplus.github.io/CWG/issues/1820.html) is
resolved by requiring that a qualified declarator-id declare an entity.

P1787R6 changes wording of [dcl.pre]/9. Quote from the current draft
([[dcl.pre]/5](https://eel.is/c++draft/dcl.pre#5)):
> If a
[declarator-id](https://eel.is/c++draft/dcl.decl.general#nt:declarator-id)
is a name, the
[init-declarator](https://eel.is/c++draft/dcl.decl.general#nt:init-declarator)
and (hence) the declaration introduce that
name[.](https://eel.is/c++draft/dcl.pre#5.sentence-1)
> [Note [3](https://eel.is/c++draft/dcl.pre#note-3): Otherwise, the
[declarator-id](https://eel.is/c++draft/dcl.decl.general#nt:declarator-id)
is a
[qualified-id](https://eel.is/c++draft/expr.prim.id.qual#nt:qualified-id)
or names a destructor or its
[unqualified-id](https://eel.is/c++draft/expr.prim.id.unqual#nt:unqualified-id)
is a [template-id](https://eel.is/c++draft/temp.names#nt:template-id)
and no name is
introduced[.](https://eel.is/c++draft/dcl.pre#5.sentence-2)
— end note]

Author: Endilll

clang/test/CXX/drs/cwg18xx.cpp

@@ -222,6 +222,53 @@ namespace cwg1815 { // cwg1815: no
 #endif
 }
 
+namespace cwg1820 { // cwg1820: 3.5
+typedef int A;
+typedef int cwg1820::A;
+// expected-warning@-1 {{extra qualification on member 'A'}}
+// expected-error@-2 {{typedef declarator cannot be qualified}}
+
+namespace B {
+typedef int cwg1820::A;
+// expected-error@-1 {{cannot define or redeclare 'A' here because namespace 'B' does not enclose namespace 'cwg1820'}}
+// expected-error@-2 {{typedef declarator cannot be qualified}}
+}
+
+class C1 {
+  typedef int cwg1820::A;
+  // expected-error@-1 {{non-friend class member 'A' cannot have a qualified name}}
+  // expected-error@-2 {{typedef declarator cannot be qualified}}
+};
+
+template <typename>
+class C2 {
+  typedef int cwg1820::A;
+  // expected-error@-1 {{non-friend class member 'A' cannot have a qualified name}}
+  // expected-error@-2 {{typedef declarator cannot be qualified}}
+};
+
+void d1() {
+  typedef int cwg1820::A;
+  // expected-error@-1 {{definition or redeclaration of 'A' not allowed inside a function}}
+  // expected-error@-2 {{typedef declarator cannot be qualified}}
+}
+
+template<typename>
+void d2() {
+  typedef int cwg1820::A;
+  // expected-error@-1 {{definition or redeclaration of 'A' not allowed inside a function}}
+  // expected-error@-2 {{typedef declarator cannot be qualified}}
+}
+
+#if __cplusplus >= 201103L
+auto e = [] {
+  typedef int cwg1820::A;
+  // expected-error@-1 {{definition or redeclaration of 'A' not allowed inside a function}}
+  // expected-error@-2 {{typedef declarator cannot be qualified}}
+};
+#endif
+} // namespace cwg1820
+
 namespace cwg1821 { // cwg1821: 2.9
 struct A {
   template <typename> struct B {

clang/www/cxx_dr_status.html

@@ -10728,7 +10728,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1820.html">1820</a></td>
     <td>CD6</td>
     <td>Qualified typedef names</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Clang 3.5</td>
   </tr>
   <tr id="1821">
     <td><a href="https://cplusplus.github.io/CWG/issues/1821.html">1821</a></td>