Fix Objective-C stub APIs on Linux

[p-x9]

Summary

• Restrict the Objective-C stub metadata initializer API to platforms with Objective-C interoperability.
• Prevent Linux consumers from emitting an unresolved swift_getObjCClassMetadata reference during final executable linking.

Root cause

The library target could compile and archive successfully on Linux, but the public @convention(c) function type returned AnyClass?. When a consumer linked the affected object code, Swift emitted a reference to Objective-C class metadata support that is unavailable in the Linux runtime.

Impact

Linux consumers can link the package without pulling in the unavailable Objective-C metadata runtime symbol. Apple-platform behavior remains unchanged.

Validation

• swift build
• Confirmed the PR contains only the canImport(ObjectiveC) guard around ObjCSwiftMetadataInitializer and initialize(in:_:).

[Copilot]

Pull request overview

This PR updates the Objective-C stub class API surface so that Objective-C metadata initialization symbols are only exposed/compiled on platforms that support Objective-C interoperability, preventing Linux consumers from linking against unavailable Objective-C runtime metadata support.

Changes:

• Wrap ObjCSwiftMetadataInitializer in #if canImport(ObjectiveC) so it is not part of the Linux build.
• Wrap ObjCStubClassProtocol.initialize(in:_:) in the same Objective-C availability guard to avoid emitting unresolved Objective-C metadata runtime references on Linux.

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