Add a feature for this new attribute, and make sure we use the feature
guard for functions that use it, e.g., the new `async`.
Finishes rdar://76927008.
Extend the checks for marker protocols and rethrows protocols to ensure
that we #if out more code that relies on them in module interface
generation. This makes the _Concurrency module parseable by much older
compilers.
Fixes rdar://75291705.
Thanks to a great idea from Slava, simplify the printing of marker
protocols in Swift interfaces while maintaining backward compatibility.
For compilers that cannot handle marker protocols, print a typealias
to `Any` instead, which is a good stand-in for just about everything.
When generating a module interface, emit `#if` around any declarations
that are tied to specific, named language features. This allows module
interfaces to be processed by older Swift compilers that do not
support these newer features, such as async/await or actors.
The amount of effort required to correctly handle a new kind of
feature varies somewhat drastically based on the feature itself. The
"simple" case is where a particular declaration can only exist if a
feature is available. For example, and `async` declaration is fairly
easy to handle; a `@_marker` protocol's conformances are not.
Fixes rdar://73326633.
