[libc++] Post-commit review on D99928.

The "user-defined conversion by implicit constructor" codepath is already
handled by `B(int)`; we don't need to test `A(const A&)` a second time
via `DA` (nor the isomorphic case with `DB`).
We don't need `&` anywhere in this test.
Generally, `operator()` should be const; this test needn't be special.
(No functional change in test coverage.)