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