An always enabled availability domain is implicitly available in all contexts,
so uses of declarations that are marked as `@available` in the domain are never
rejected. This is useful for an availability domain representing a feature flag
that has become permanently enabled.
Partially resolves rdar://157593409.
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.