KT-3159 Disallow overriding var with different type #KT-3159 fixed

compiler/testData/diagnostics/tests/subtyping/kt3159.kt

@@ -0,0 +1,9 @@
+trait Super {
+    var v: CharSequence
+    val v2: CharSequence
+}
+
+class Sub: Super {
+    override var v: <!PROPERTY_TYPE_MISMATCH_ON_OVERRIDE!>String<!> = "fail"
+    override val v2: String = "ok"
+}