Minor, add regression test for K1 issue KT-61101

compiler/testData/diagnostics/tests/properties/PrivateSetInSuperclass.fir.kt

@@ -0,0 +1,13 @@
+// ISSUE: KT-61101
+
+open class A {
+    var x: Int = 0
+        private set
+}
+
+class B : A()
+
+fun test() {
+    val b = B()
+    b.<!INVISIBLE_SETTER!>x<!> = throw Exception()
+}

compiler/testData/diagnostics/tests/properties/PrivateSetInSuperclass.kt

@@ -0,0 +1,13 @@
+// ISSUE: KT-61101
+
+open class A {
+    var x: Int = 0
+        private set
+}
+
+class B : A()
+
+fun test() {
+    val b = B()
+    b<!UNREACHABLE_CODE!>.x =<!> throw Exception()
+}