ConstExtract: Refactor handling of `AvailabilitySpec`.
Soon, `AvailabilitySpec` will require that the `AvailabiltyDomain` it contains
be queried using a request that takes the `DeclContext` as input in order to
resolve the parsed domain name to an instance of `AvailabilityDomain`. The
constant extraction pipeline needed a bit of refactoring to thread a
`DeclContext` through to the place where it will be needed to execute the
query.
NFC.
