Treat `@_unavailableInEmbedded` as if it were `@available(Embedded,
unavailable)` and apply platform compatibility logic in the availability
checker. Revert back to disallowing calls to universally unavailable functions
(`@available(*, unavailable)`) in all contexts.
Availability checking for types was only suppressed when the immediate context
for the use of the type was explicitly marked unavailable. Availability is
lexical so the checking should be suppressed in the entire scope instead.
Implement a query that returns the `AvailabilityContext` for a given
`SourceLoc` and `DeclContext`. Re-implement the existing type checker query
that just returns an `AvailabilityRange` on top of the new query.
Make the pointer to uniqued storage an implementation detail of an
`AvailabilityContext` value. This way clients of `AvailabilityContext` don't
need to think about pointers and can have access to mutating operations on a
context when appropriate.
