Update availability diagnostic for macOS major version number changes

Resolves: SR-15093

Author: cbjeukendrup

lib/Sema/TypeCheckAvailability.cpp

@@ -1539,19 +1539,20 @@ static bool fixAvailabilityByNarrowingNearbyVersionCheck(
       TRC && TRC->getReason() != TypeRefinementContext::Reason::Root) {
 
     // Only fix situations that are "nearby" versions, meaning
-    // disagreement on a minor-or-less version for non-macOS,
-    // or disagreement on a subminor-or-less version for macOS.
+    // disagreement on a subminor-or-less version for macOS 10,
+    // or disagreement on a minor-or-less version otherwise.
     auto RunningVers = RunningRange.getLowerEndpoint();
     auto RequiredVers = RequiredRange.getLowerEndpoint();
     auto Platform = targetPlatform(Context.LangOpts);
     if (RunningVers.getMajor() != RequiredVers.getMajor())
       return false;
     if ((Platform == PlatformKind::macOS ||
          Platform == PlatformKind::macOSApplicationExtension) &&
+        RunningVers.getMajor() == 10 &&
         !(RunningVers.getMinor().hasValue() &&
           RequiredVers.getMinor().hasValue() &&
           RunningVers.getMinor().getValue() ==
-          RequiredVers.getMinor().getValue()))
+              RequiredVers.getMinor().getValue()))
       return false;
 
     auto FixRange = TRC->getAvailabilityConditionVersionSourceRange(