[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/controlFlowAnalysis/privateSetter.fir.kt

@@ -0,0 +1,17 @@
+open class X(s : String) {
+    public var n: String = s
+        private set
+
+}
+
+class Z : X("subclass") {
+    fun print(): String {
+        n = n
+        return n;
+    }
+}
+
+
+fun box() : String {
+    return Z().print() //error
+}