When emitting SIL for `if #available(Swift ..., *)` queries, call the new
`_isSwiftRuntimeVersionAtLeast()` function in the stdlib to check the
condition. To support back deployment, the implementation of
`_isSwiftRuntimeVersionAtLeast()` is `@_alwaysEmitIntoClient` and performs its
comparison against the result of `_SwiftStdlibVersion.current`, which is
pre-existing ABI that the stdlib exposes for querying the Swift runtime
version.
Resolves rdar://162726037.
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.
