C-Protonation of Adamantylphosphaacetylene (1-AdC.tplbond.P) and tert-Butylphosphaacetylene (tBuC.tplbond.P) in Superacids: Phosphavinyl Cation Generation and Trapping To Form Phosphaalkenes, Formation of Isomeric Boron-Containing Spirocyclic Betaines by Reaction of 1-AdC.tplbond.P with B(OTf)3, and Theoretical Studies on Protonation of MeC.tplbond.P