[clang] Add test for CWG1821

[Endill]

Summary

P1787: My clarification in “Where can namespace-scope functions and variables be redeclared?” is applied, resolving CWG1821.
Wording: (If the declaration is not a friend declaration:) If the id-expression in a declarator-id is a qualified-id Q, let S be its lookup context; the declaration shall inhabit a namespace scope. ([dcl.meaning]/1)

[shafik]

LGTM

clang/test/CXX/drs/dr18xx.cpp

#if __cplusplus >= 201402L
   struct A { int &&r = 0; }; // expected-note {{default member init}}
   A a = {}; // FIXME expected-warning {{not supported}}
 
   struct B { int &&r = 0; }; // expected-error {{binds to a temporary}} expected-note {{default member init}}
   B b; // expected-note {{here}}
 #endif
 }
 
+namespace dr1821 { // dr1821: yes
+struct A {
+  template <typename> struct B {
+    void f();
+  };
+  template <typename T> void B<T>::f(){};
+  // expected-error@-1 {{non-friend class member 'f' cannot have a qualified name}}
+
+  struct C {
+    void f();
+  };
+  void C::f() {}
+  // expected-error@-1 {{non-friend class member 'f' cannot have a qualified name}}
+};
+} // namespace dr1821
+
 namespace dr1822 { // dr1822: yes
 #if __cplusplus >= 201103L
   int a;
   auto x = [] (int a) {
 #pragma clang __debug dump a // CHECK: ParmVarDecl
   };
 #endif
 }

clang/www/cxx_dr_status.html

<tr id="1820">
     <td>CD6</td>
     <td>Qualified typedef names</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1821">
     <td><a href="https://cplusplus.github.io/CWG/issues/1821.html">1821</a></td>
     <td>CD6</td>
     <td>Qualified redeclarations in a class <I>member-specification</I></td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="1822">
     <td><a href="https://cplusplus.github.io/CWG/issues/1822.html">1822</a></td>
     <td>CD6</td>
     <td>Lookup of parameter names in <I>lambda-expression</I>s</td>
     <td class="full" align="center">Yes</td>
   </tr>
   <tr id="1823">