[CSRanking] Amend concrete-over-protocol logic to only do an is-a check

As discussed in swiftlang#18951, we don't want to be comparing two different protocol decls here, as that could lead to spurious 'is more specialized' results. I'm not aware of any case that currently trips this logic up though, so no tests to accompany.

Author: hamishknight

lib/Sema/CSRanking.cpp

@@ -471,10 +471,10 @@ static bool isDeclAsSpecializedAs(TypeChecker &tc, DeclContext *dc,
       //    }
       //
       if (tc.Context.isSwiftVersionAtLeast(5) && !isDynamicOverloadComparison) {
-        auto *proto1 = dyn_cast<ProtocolDecl>(outerDC1);
-        auto *proto2 = dyn_cast<ProtocolDecl>(outerDC2);
-        if (proto1 != proto2)
-          return proto2;
+        auto inProto1 = isa<ProtocolDecl>(outerDC1);
+        auto inProto2 = isa<ProtocolDecl>(outerDC2);
+        if (inProto1 != inProto2)
+          return inProto2;
       }
 
       Type type1 = decl1->getInterfaceType();