This experimental feature will be used to force the compiler to treat `Swift`
runtime availability as separate from platform availability when compiling for
targets that have the Swift runtime built-in.
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.
