Synthesized 'copy' in data classes cannot override anything since 1.3

Synthesized 'copy' introduces default values for parameters, which is
prohibited for regular overrides.
Report warning in language version 1.2-, error in 1.3+.

compiler/testData/codegen/box/dataClasses/copy/kt12708.kt

@@ -1,3 +1,5 @@
+// LANGUAGE_VERSION: 1.1
+
 fun box(): String {
     val a: A = B(1)
     a.copy(1)