swift

e9053099 - Parse/Sema: Move some `AvailabilitySpec` diagnostics from Parse to Sema.

Parse/Sema: Move some `AvailabilitySpec` diagnostics from Parse to Sema. Eventually, querying the `AvailabilityDomain` associated with an `AvailabilitySpec` will require invoking a request that takes a `DeclContext`. This means that any diagnostics related to the domain identified by an `AvailabilitySpec` need to be emitted during type-checking rather than parsing. This change migrates several `AvailabilitySpec` diagnostics from Parse to Sema to unblock further work.