Add support for the `Swift` availability domain, which represents availability
with respect to the Swift runtime. Use of this domain is restricted by the
experimental feature `SwiftRuntimeAvailability`.
When importing custom availability domains with dynamic predicates from Clang
modules, synthesize predicate functions for `if #available` queries and call
them when generating SIL.
Resolves rdar://138441312.
Generalize SILGen for `if #available` checks by delegating the determination of
the query function and its arguments to `AvailabilityQuery`. Update SILGen for
`@backDeployed` thunks to use the same infrastructure.
NFC.
