[5.10] AST: Fix macCatalyst availability for synthesized declarations

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• Explanation: The macCatalyst platform availability of a declaration may be inferred from the iOS platform availability of that declaration in the absence of an explicit macCatalyst availability attribute. This means that when inheriting macCatalyst platform availability, the explict iOS availability of an inner ancestor should take precedence over explicit macCatalyst availability of an outer ancestor. The algorithm that computes inferred availability attributes for synthesized declarations was treating macCatalyst and iOS as independent platforms, though, sometimes yielding inapproparite macCatalyst availability.
• Radar: rdar://107766644
• Scope: This PR changes the algorithm for generating availability annotations for synthesized declarations. Affected kinds of declarations include the unownedExecutor requirement for actors, implicit _read and _modify accessors, and inherited designated initializers. The change to the algorithm is narrow in that it only suppresses the addition of @available attributes for platforms that inherit availability from other root platforms. Today, the only platforms that meet that criteria are macCatalyst and the platform Extension variants.
• Risk: Low. Incorrect synthesis of availability attributes could make some derived declarations available when they should not be, leading to missing symbol crashes at runtime when running back deployed apps that reference the inappropriately available. However, the core functionality of existing availability inference behavior is well tested and the scope of the change is limited to less common platforms.
• Testing: Added tests for synthesis of unownedExecutor decls that, without the fix, demonstrated inappropriate macCatalyst availability.
• Reviewed By: @xymus, @nkcsgexi, @artemcm
• main PR: AST: Fix macCatalyst availability for synthesized declarations #70533

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@swift-ci please test