Sema: Warn on all non-resilient uses of @_implementationOnly import, even for clang targets

[xymus]

The warning about using '@_implementationOnly' without enabling library evolution for 'client' may lead to instability during execution was wrongly restricted to only imports of Swift modules. Make sure they are raised for imports of clang modules as well.

rdar://135233043

[xymus]

@swift-ci Please smoke test

[xymus]

@swift-ci Please smoke test

I reviewed the new logic to keep the warning to use internal import instead of @_implementationOnly to imports of Swift modules only. This warning can be noisy, limiting it may prevent users from outright learning to ignore it. Typical uses of an @_implementationOnly import of a clang module is often for a project internal module instead of a wider layering concern as we have with Swift module targets. We can leave legacy uses of @_implementationOnly in peace for now.

[tshortli]

The potential miscompiles that can arise from @_implementationOnly are equally present whether it is a Swift or a clang module that is being imported, though. I thought we wanted this warning to help prevent more time wasted debugging that kind of issue, for both project owners and compiler engineers.

[xymus]

@swift-ci Please smoke test

[xymus]

@tshortli My last change only silenced the deprecation warning in resilient modules. That's generally reliable, the one miscompile there I'm aware of would be with test clients that can't load the dependency because of different search paths. It's rare but maybe it's worth the warning.

[tshortli]

Oh, I misunderstood, thanks for clarifying. I have debugged miscompiles stemming from the combination of @_implementationOnly import and @testable imports of resilient libraries, but I agree it's way more niche and perhaps not worth the noise. I think eventually we should warn about it, though.