Data Outflow: Allow for-loop variables

#KT-19018 Fixed

idea/testData/slicer/outflow/forVariable.kt

@@ -0,0 +1,9 @@
+// FLOW: OUT
+
+fun f1(param: String) {}
+
+fun f4(list: List<String>) {
+    for (<caret>s in list)
+        if (s.isNotEmpty())
+            f1(s)
+}

idea/testData/slicer/outflow/forVariable.results.txt

@@ -0,0 +1,4 @@
+6 for (<bold>s</bold> in list)
+7 	if (<bold>s</bold>.isNotEmpty())
+8 	f1(<bold>s</bold>)
+3 		fun f1(<bold>param: String</bold>) {}