SwiftTailCC: teach verifier musttail rules applicable to this CC.

SwiftTailCC has a different set of requirements than the C calling convention
for a tail call. The exact argument sequence doesn't have to match, but fewer
ABI-affecting attributes are allowed.

Also make sure the musttail diagnostic triggers if a musttail call isn't
actually a tail call.

GitOrigin-RevId: 9ff2eb1ea596a52ad2b5cfab826548c3af0a1e6e