Silence compiler by adding cosmetic virtual destructors.

[SVN r41549]

test/implicit.cpp

@@ -24,6 +24,7 @@ X make_x(int n) { return X(n); }
 struct bar {};
 struct foo
 {
+    virtual ~foo() {}; // silence compiler warnings
     virtual void f() = 0;
     operator bar() const { return bar(); }
 };