[6.0][Concurrency] Don't ignore mismatching isolation for overrides of Clang-imported superclass methods.

[hborla]

• Explanation: The actor isolation checker was ignoring mismatching isolation for overrides if the superclass is imported from ObjC. However, mismatching isolation for overrides is invalid, and there's no reason to ignore this mistake for ObjC-imported superclasses. This change is staged in behind complete concurrency checking. This change also removes a bogus fix-it to add global actor isolation to an override if it synchronously calls a global actor isolated method, which won't work.
• Scope: Only impacts overrides where the superclass is imported from ObjC, where the override is isolated to a global actor and the superclass method is not.
• Issue: rdar://124409467, adding global actor erroneously allowed when overriding an ObjC class's method #72238
• Risk: Low; the change is staged in as a warning under -strict-concurrency=complete.
• Testing: Added a new test.
• Reviewer: TBD
• Main branch PR: [Concurrency] Don't ignore mismatching isolation for overrides of Clang-imported superclass methods. #74237

[hborla]

@swift-ci please test