An `AvailableAttr` written in source with an unrecognized availability domain
is now only marked invalid after type-checking the attribute. This resulted in a
regression where `CaseIterable` synthesis was blocked incorrectly under the
following very narrow circumstances:
1. Every `@available` attribute on the elements of the enum is invalid.
2. The module is being emitted and lazy type-checking is enabled.
3. The enum is public and the only top-level declaration in the file.
Type-checking the attribute was delayed just enough that it would not be
considered invalid by the type the `CaseIterable` conformance was being
synthesized, resulting in a spurious error.
There were zero tests exercising `CaseIterable` synthesis for enums with
elements that have availability requirements, so I added some.
Resolves rdar://144897917.
