When the CustomAvailability experimental feature is enabled, make it an error
to specify an unrecognized availability domain name. Also, add these
diagnostics to a diagnostic group so that developers can control their behavior
when they are warnings.
Resolves rdar://152741624.
Delay resolution of availability domain identifiers parsed in availability
specifications until type-checking. This allows custom domain specifications to
be written in `if #available` queries.
Implement lookup of availability domains for identifiers on
`AvailabilityDomainOrIdentifier`. Add a bit to that type which represents
whether or not lookup has already been attempted. This allows both
`AvailableAttr` and `AvailabilitySpec` to share a common implementation of
domain lookup.
