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/diagnostics/tests/dataClasses/dataClassExplicitlyOverridingCopyWithDefaults.kt

@@ -0,0 +1,9 @@
+// !LANGUAGE: +ProhibitDataClassesOverridingCopy
+
+interface WithCopy<T> {
+    fun copy(str: T): WithCopy<T>
+}
+
+<!DATA_CLASS_OVERRIDE_DEFAULT_VALUES_ERROR!>data<!> class <!CONFLICTING_JVM_DECLARATIONS, CONFLICTING_JVM_DECLARATIONS, CONFLICTING_JVM_DECLARATIONS!>Test(val str: String)<!> : WithCopy<String> {
+    <!CONFLICTING_OVERLOADS!>override fun copy(str: String = <!DEFAULT_VALUE_NOT_ALLOWED_IN_OVERRIDE!>this.str<!>)<!> = Test(str)
+}