TypeCheckType: Fix existential any migration diagnostics for extra-modular protocols

[AnthonyLatsis]

There are two issues:

• The evaluation of ExistentialRequiresAnyRequest is dependent on whether the ExistentialAny feature is enabled in the current compiler invocation, but the evaluation is bypassed for deserialized protocols because the result of the request is part of the serialized record. This made existentialRequiresAny() independent of the feature state for deserialized protocols (Enabling upcoming feature ExistentialAny does not work for standard library protocols. #65034).
• The evaluation of ExistentialRequiresAnyRequest must not depend on the feature state, or else is will persist in module files and override the feature state for protocols originating in those modules. In other words, the feature must not be enforced on protocols originating in a different module just because that module was compiled with the feature enabled.

Resolves #65034

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IMO there is no reason not to accept this into 5.9, but I would also vote to take it into 5.8 because the issue affects the new migration experience for a pretty impactful upcoming feature.

[hborla]

You could also pass in the usage context into existentialRequiresAny so the method itself accounts for language feature state, but the ExistentialRequiresAny request does not. Thoughts?

[AnthonyLatsis]

I was going to but that is what the serializer calls to obtain the evaluation result, which is somewhat misleading in my opinion. I think we should rename the request to something like HasSelfOrAssociatedTypeRequirements and pair it with a method to use in the serializer so that existentialRequiresAny could account for language feature state.

[hborla]

👍 that sounds like a good approach to me (and we don't need to do that as part of this PR)

[hborla]

Thank you!

[AnthonyLatsis]

@swift-ci please smoke test

[hborla]

[...]
forward_trailing_closure_errors.swift -swift-version 6
--
Exit Code: 1

Command Output (stderr):
--
/Users/ec2-user/jenkins/workspace/swift-PR-macos-smoke-test/branch-main/swift/test/expr/postfix/call/forward_trailing_closure_errors.swift:23:14: error: unexpected error produced: use of protocol 'Error' as a type must be written 'any Error'
  onError: ((Error) -> Void)? = nil,
             ^

Hah. That's true!

[AnthonyLatsis]

@swift-ci please smoke test

[hborla]

@AnthonyLatsis will you please cherry-pick this to release/5.9 and ping me for branch manager approval?

[AnthonyLatsis]

Sure. I was waiting for a green light on #65132 so that I can cherry-pick it as well. I’ve got more related fixes coming up and I would prefer them to layer up cleanly on both branches.