Weakly import declarations with conditional availability

[slavapestov]

Fixes rdar://46674512.

[slavapestov]

@swift-ci Please test

[slavapestov]

@swift-ci Please test source compatibility

[jrose-apple]

I don't think this is the correct approach. You need to weak-reference things if your deployment target isn't new enough; it doesn't matter what the use site's AvailabilityContext is. Therefore it doesn't need to be an extra parameter.

[slavapestov]

It’s not passing in the use site’s local AvailabilityContext. It’s always passing in the global one from the ASTContext. I guess I could get rid of the extra parameter but I’m not a huge fan of all the global state in the ASTContext in general.

[jrose-apple]

I agree about the global state but in this case I think it's the lesser of two evils. Having the parameter encourages people to pass the wrong thing, especially if/when we start using AvailabilityContext in SIL.

[slavapestov]

@jrose-apple OK then, I'll clean that up. Other than changing it to always use the global AvailabilityContext do you think this is the right approach?

[jrose-apple]

Yeah, I think this is reasonable. If we wanted to handle backwards-deployment-without-availability or directly testing symbols for presence/absence it'd be more difficult, but for what we need right now this makes sense. (And the explicit @_weakLinked is close to handling the rest.)

[jrose-apple]

Conformances might be tricky, though, since those aren't Decls.