[6.1]Revert [ConformanceLookup] Don't allow skipping inherited unavailable conformances in favor of explicit available ones.

[hborla]

• Explanation: [ConformanceLookup] Don't allow skipping inherited unavailable conformances in favor of explicit available ones. #75135 exposed a few incorrect Sendable annotations in libraries. Most notably, Formatter in Foundation has an unavailable conformance to Sendable, while subclasses including DateFormatter have an available conformance to Sendable. This is not supported; types can only have one conformance to a protocol. If a superclass wants to allow subclasses to have differing availability on a particular protocol conformance, the subclass should have no conformance to that protocol at all. The fix is to remove the unavailable Sendable conformance on Formatter in Foundation. This revert is a workaround until libraries have time to update their conformances.

  This impact is programmers will miss out on redundant Sendable conformance warnings, and observe unexpected behavior for code that declares both an unavailable conformance and an available conformance to Sendable (which is already the case in Swift 6.0).

  Note that I did not revert [Concurrency] Don't warn about re-stating inherited unavailable conformances to Sendable. #75359. It should not have any impact. The Swift 6.0 compiler has the exact same set of reverts.

• Scope: Only impacts redundant conformances to Sendable where at least one of the conformances is unavailable.

• Issues: Swift 6.1 regression in Sendable checking #78635

• Original PRs: There is no main PR for this revert. This revert is for release/6.1 only. The compiler change is correct, but there are a handful of incorrect Sendable annotations in libraries that should be fixed before this change lands to avoid source compatibility issues in client projects.

• Risk: Low; this change restores the behavior before [ConformanceLookup] Don't allow skipping inherited unavailable conformances in favor of explicit available ones. #75135 and [Conformance] Always downgrade redundant conformances to marker protocols to a warning. #75425. These changes were reverted from release/6.0 for a different reason

• Testing: N/A

• Reviewers: @bnbarham @DougGregor

[hborla]

@swift-ci please test

[hborla]

@swift-ci please test source compatibility

[hborla]

@swift-ci please test macOS