CFG: provide "used as expression" in enum constant constructor

This allows to avoid
"unused return value of a function with lambda expression body"
for such a situation
So #KT-20429 Fixed

idea/testData/inspectionsLocal/unusedLambdaExpressionBody/.inspection

@@ -0,0 +1 @@
+org.jetbrains.kotlin.idea.inspections.UnusedLambdaExpressionBodyInspection

idea/testData/inspectionsLocal/unusedLambdaExpressionBody/inEnumEntry.kt

@@ -0,0 +1,7 @@
+// PROBLEM: none
+
+enum class Test(f: () -> Unit) {
+    A(<caret>getFunc())
+}
+
+fun getFunc(): () -> Unit = {}