[clang] Add test for CWG1960

[Endill]

Summary

P1787: "CWG1960 (currently closed as NAD) is resolved by removing the rule in question (which is widely ignored by implementations and gives subtle interactions between using-declarations)."
Wording: "In a using-declarator that does not name a constructor, every declaration named shall be accessible."

[shafik]

LGTM

clang/test/CXX/drs/dr19xx.cpp

#if __cplusplus >= 201103L
   };
   // FIXME: As a resolution to an open DR against P0136R0, we disallow
   // use of inherited constructors to construct from a single argument
   // where the base class is reference-related to the argument type.
   c q(static_cast<c&&>(q));
 #endif
 }
 
+namespace dr1960 { // dr1960: no
+struct A {
+void f() {}
+protected:
+void g() {}
+};
+
+struct B: A {
+private:
+using A::f;
+using A::g;
+};
+
+struct C : B {
+// FIXME: both declarations are ill-formed, because A::f and A::g
+// are not accessible.
+using A::f;
+using A::g;
+};
+}
+
 namespace dr1966 { // dr1966: 11
 #if __cplusplus >= 201103L
   struct A {
     enum E : int {1}; // expected-error {{expected identifier}} (not bit-field)
   };
   auto *p1 = new enum E : int; // expected-error {{only permitted as a standalone declaration}}
   auto *p2 = new enum F : int {}; // expected-error {{cannot be defined in a type specifier}}
   auto *p3 = true ? new enum G : int {}; // expected-error {{forward reference}} expected-error {{incomplete}} expected-note {{declaration}}

clang/www/cxx_dr_status.html

<tr id="1959">
     <td>CD4</td>
     <td>Inadvertently inherited copy constructor</td>
     <td class="full" align="center">Clang 3.9</td>
   </tr>
   <tr id="1960">
     <td><a href="https://wg21.link/cwg1960">1960</a></td>
     <td>NAD</td>
     <td>Visibility of entity named in class-scope <I>using-declaration</I></td>
-    <td class="none" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr id="1961">
     <td><a href="https://wg21.link/cwg1961">1961</a></td>
     <td>C++17</td>
     <td>Potentially-concurrent actions within a signal handler</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr class="open" id="1962">