The "#if compiler(>=5.3) && $AsyncAwait" checks were necessary for
staging in concurrency in Swift 5.5. At this point, it's safe to assume
that any compiler that tries to read a generated Swift interface file will
support concurrency, so we can stop emitting these guards.
Some decls that are expected to be synthesized for distributed actors are
printed explicitly in swiftinterfaces so diagnostics and assertions need to
take that possibility into account.
Resolves rdar://108533918
