Revert [6.0][ConformanceLookup] Don't allow skipping inherited unavailable conformances in favor of explicit available ones.

[hborla]

• Explanation: [6.0][ConformanceLookup] Don't allow skipping inherited unavailable conformances in favor of explicit available ones. #75223 exposed a crash in region analysis, presumably because it changed which Sendable conformance is found when there is an inherited, unavailable conformance to Sendable. I suspect the nuanced behavior of @_nonSendable played a role. Instead of changing the behavior of @_nonSendable, which I tried not to do in the original PR, I'm reverting this change for 6.0. This change can be landed later, because it only introduces warnings.

  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 5.10).

  Note that I did not revert [6.0][Concurrency] Don't warn about re-stating inherited unavailable conformances to Sendable. #75360. It should not have any impact.

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

• Issues: rdar://132347404

• Original PRs: N/A, this revert is for release/6.0 only.

• Risk: Low; this change restores the behavior before [6.0][ConformanceLookup] Don't allow skipping inherited unavailable conformances in favor of explicit available ones. #75223 and [6.0][Conformance] Always downgrade redundant conformances to marker protocols to a warning. #75426.

• Testing: N/A

• Reviewers: N/A

[hborla]

@swift-ci please test