Fix UB in one libcxx test, when deleting D through a pointer to B.

This undefined behavior was found by applying Lénárd Szolnoki's proposal
to disable implicit conversion of default_delete<D> to default_delete<B>.

The offending part of the test is circa line 243.

The wording that makes it undefined behavior is http://eel.is/c++draft/expr.delete#3 .

Differential Revision: https://reviews.llvm.org/D90536

libcxx/test/std/utilities/optional/optional.object/optional.object.assign/optional_U.pass.cpp

@@ -51,7 +51,7 @@ struct Y2
     Y2& operator=(const int&) { return *this; }
 };
 
-class B {};
+struct B { virtual ~B() = default; };
 class D : public B {};