[6.0][Concurrency] Don't warn about re-stating inherited unavailable conformances to Sendable.

[hborla]

• Explanation: The compiler warns on classes that inherit @unchecked Sendable conformances and don't re-state the conformance so that the programmer is aware that they're responsible for upholding the promises of Sendable without the compiler's help. This checking was completely skipped for inherited, unavailable Sendable conformances until [6.0][ConformanceLookup] Don't allow skipping inherited unavailable conformances in favor of explicit available ones. #75223. This exposed a bug where the unavailable check was not done on the root conformance, which lead to bogus warnings that subclasses must restate the @unchecked Sendable conformance:

  @available(*, unavailable)
  extension Foo : @unchecked Sendable { }
  
  class Foo {}
  
  class Bar: Foo {} // warning: class 'Bar' must restate inherited '@unchecked Sendable' conformance

  The fix is to look for unavailable attributes on the root conformance's decl context.

• Scope: Only impacts inherited, unavailable conformances to Sendable.

• Issues: rdar://132059160

• Original PRs: [Concurrency] Don't warn about re-stating inherited unavailable conformances to Sendable. #75359

• Risk: Low; this change effectively replaces a conformance->getDeclContext() argument with conformance->getRootConformance()->getDeclContext(), and the only effect is skipping warnings.

• Testing: Added a new test exercising the lack of warning.

• Reviewers: TBD

[hborla]

@swift-ci please test