[Sema] Prioritize unavailability check for protocol witnesses

The errors on explicit unavailability are more precise than those on
availablity version. Make sure we prioritize the unavailability ones
first.

Author: xymus

lib/Sema/TypeCheckProtocol.cpp

@@ -1469,6 +1469,11 @@ RequirementCheck WitnessChecker::checkWitness(ValueDecl *requirement,
       return CheckKind::UsableFromInline;
   }
 
+  if (match.Witness->getAttrs().isUnavailable(getASTContext()) &&
+      !requirement->getAttrs().isUnavailable(getASTContext())) {
+    return CheckKind::WitnessUnavailable;
+  }
+
   auto requiredAvailability = AvailabilityContext::alwaysAvailable();
   if (checkWitnessAvailability(requirement, match.Witness,
                                &requiredAvailability)) {
@@ -1498,11 +1503,6 @@ RequirementCheck WitnessChecker::checkWitness(ValueDecl *requirement,
     }
   }
 
-  if (match.Witness->getAttrs().isUnavailable(getASTContext()) &&
-      !requirement->getAttrs().isUnavailable(getASTContext())) {
-    return CheckKind::WitnessUnavailable;
-  }
-
   return CheckKind::Success;
 }