SourceKit: do not print implementation-only imports

[egorzhdan]

Implementation-only imports are unnecessary in generated module interfaces, since they don't have an effect on the module's public interface:

• implementation-only imports aren't exported to the module's dependencies
• module's public API cannot reference symbols imported as implementation-only.

This change hides @_implementationOnly imports from generated Swift module interfaces.

It allows, for example, to hide import Darwin/import WinSDK/import Glibc statements from SwiftPM's PackageDescription library interface.

[egorzhdan]

@swift-ci please smoke test

[CodaFi]

This looks quite reasonable, thank you. I would love it if we could have a dedicated test under test/IDE that checks to make sure we print @_implementationOnly, @_exported, and @_private but it can be done in a follow-up PR.

[egorzhdan]

@CodaFi hmm, not sure I understand it correctly – there's already a test checking that @_implementationOnly isn't printed in the generated module interface (test/Serialization/Recovery/implementation-only-override.swift) and a few tests for the @_exported attribute (for example, test/IDE/print_clang_framework_with_overlay.swift).

implementation-only-override.swift uses swift-ide-test, perhaps it could have been placed under test/IDE as well.

Is there an untested part that I'm missing?

[CodaFi]

The test under Serialization is trying to make sure that overrides from implementation-only imports can be serialized without issue and show up in the generated interface. I would like a dedicated test for just the expectation that attribute-qualified imports show up in the generated interface - to keep our concerns well separated.

[egorzhdan]

Ah, okay, sounds fair to me. I've sent a new PR with a test: #35706