ASTPrinter: print public inherited protocols of the skipped private protocols

[egorzhdan]

When printing the list of inherited protocols in the module interface, if private stdlib protocols are requested to be hidden (PrintOptions::SkipUnderscoredStdlibProtocols), make sure to print public inherited protocols of the hidden protocols.

This is especially noticeable for NSError types imported into Swift. For example, CocoaError is imported with a synthesized conformance to _BridgedStoredNSError, however, in the module interface generated by SourceKit, _BridgedStoredNSError is currently hidden from the inheritance list:

/// Describes errors within the Cocoa error domain.
public struct CocoaError {
    // ...
}

There is no indication in the module interface that CocoaError is an Error and can be thrown. This might cause confusion for someone reading the module interface, and for static analysis tools relying on it.

This change improves the module interface generation for synthesized conformances, so that the inheritance list contains all the public conformances that can be observed by the user. For CocoaError, we would now see this:

/// Describes errors within the Cocoa error domain.
public struct CocoaError : CustomNSError, Hashable, Error {
    // ...
}

[egorzhdan]

@swift-ci please smoke test

[egorzhdan]

@nkcsgexi could you please take a look at this PR?
This is somewhat related to your change from a few years ago (#14248).

[zoecarver]

Is it possible that one of these protocols would be hidden?

[egorzhdan]

Yes, in that case we'll just skip it on L5764. This actually happens with the NSError types: they are imported with a conformance to _BridgedStoredNSError (hidden), which inherits from _ObjectiveCBridgeableError (also hidden), so this logic will look into the parent protocol of _ObjectiveCBridgeableError, which is Error, and the conformance to Error will be printed.

[zoecarver]

Thanks Egor.