Prot#VX11-661-101: A Phase 2, Multicenter, Double-Blinded, Placebo-Controlled, 3-Part Study to Evaluate Safety, Efficacy, Pharmacokinetics, and Pharmacodynamics of VX-661 Monotherapy and VX-661/VX-770 Cotherapy in Subjects with Cystic Fibrosis, Homozygous