[clang] Add test for CWG607

[Endill]

Summary

P1787: CWG607 is resolved by looking up unqualified names in a mem-initializer-id from outside the parameter scope.
Wording: Lookup for an unqualified name in a mem-initializer-id ignores the constructor’s function parameter scope. ([class.base.init]/2)

[shafik]

Thank you for working on these DRs, LGTM

clang/test/CXX/drs/dr6xx.cpp

void test(S<int> s) {
     h(s); // expected-error {{no match}}
 
     g(test);
     h(test); // ok, an rvalue reference can bind to a function lvalue
   }
 #endif
 }
 
+namespace dr607 { // dr607: yes
+namespace example1 {
+struct Y {};
+
+template <typename T> struct X : public virtual Y {};
+
+template <typename T> class A : public X<T> {
+  template <typename S> A(S) : S() {}
+};
+
+template A<int>::A(Y);
+} // namespace example1
+
+namespace example2 {
+namespace N {
+struct B {
+  B(int);
+};
+typedef B typedef_B;
+struct D : B {
+  D();
+};
+} // namespace N
+
+N::D::D() : typedef_B(0) {}
+} // namespace example2
+} // namespace dr607
+
 namespace dr608 { // dr608: yes
   struct A { virtual void f(); };
   struct B : A {};
   struct C : A { void f(); };
   struct D : B, C {};
 }
 
 int dr610[-0u == 0u ? 1 : -1]; // dr610: yes

clang/www/cxx_dr_status.html

<tr id="606">
     <td>CD1</td>
     <td>Template argument deduction for rvalue references</td>
     <td class="full" align="center">Yes</td>
   </tr>
   <tr id="607">
     <td><a href="https://wg21.link/cwg607">607</a></td>
     <td>CD6</td>
     <td>Lookup of <I>mem-initializer-id</I>s</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="608">
     <td><a href="https://wg21.link/cwg608">608</a></td>
     <td>CD2</td>
     <td>Determining the final overrider of a virtual function</td>
     <td class="full" align="center">Yes</td>
   </tr>
   <tr id="609">