Put AvailabilityRange into its own header with very few dependencies so that it
can be included freely in other headers that need to use it as a complete type.
NFC.
When a declaration is `@unsafe`, don't emit strict safety diagnostics
for uses of unsafe entities, constructs, or types within it. This
allows one to account for all unsafe behavior in a module using strict
memory safety by marking the appropriate declarations `@unsafe`.
Enhance the strict-safety diagnostics to suggest the addition of
`@unsafe` where it is needed to suppress them, with a Fix-It. Ensure
that all such diagnostics can be suppressed via `@unsafe` so it's
possible to get to the above state.
Also includes a drive-by bug fix where we weren't diagnosing unsafe
methods overriding safe ones in some cases.
Fixes rdar://139467327.
It replaces `DeclAttr::getUnavailable()` and `AvailableAttr::isUnavailable()`
as the designated way to query for the attribute that makes a decl unavailable.
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.
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.
