[5.9.0][Macros] Always consider pre-macro-expansion conformances as subsumed by other conformance entry kinds, before considering availability.

[hborla]

• Explanation: Extension macros that add conformances will have a placeholder ConformanceEntry so that the compiler can reason about which conformances will be added prior to macro expansion, and an explicit ConformanceEntry is added after the macro is expanded. However, the ranking between these conformances was considering availability before the conformance entry kind; if the macro-expanded conformance is less available than the always-available placeholder entry, the placeholder entry would be chosen. This lead to the conformance being dropped on the floor in serialization and SILGen, because the placeholder entry is just a placeholder, and when we create a NormalProtocolConformance for it, it's not complete. The fix is to swap the order of the availability check and the check for pre-macro-expansion placeholder conformance entries.
• Scope: Only impacts extension macros that introduce conformances.
• Risk: Very low. The change only swaps the order of two existing checks inside ConformanceLookupTable::compareConformances.
• Testing: Added a new test. Without this change, the test results in a linker error (because SILGen dropped the macro-introduced conformance).
• Issue: rdar://113569289
• Reviewer: @slavapestov
• Main branch PR: [Macros] Always consider pre-macro-expansion conformances as subsumed by other conformance entry kinds, before considering availability. #67882
• 5.9 PR: [5.9][Macros] Always consider pre-macro-expansion conformances as subsumed by other conformance entry kinds, before considering availability. #67883

[hborla]

@swift-ci please test