If participant is enrolled in Part 1, the dose of alpelisib participant receives will depend on when participant joins this study. The first group of participants will receive the lowest dose level of alpelisib and enzalutamide. Each new group will receive a higher dose of alpelisib and enzalutamide than the group before it, if no intolerable side effects were seen. Up to 5 doses of alpelisib will be tested. This will continue until the highest tolerable dose of alpelisib and enzalutamide is found.