FIR IDE: Offer AddExclExclCallFix for nullable types with an

`iterator()` function that does NOT have `operator` modifier.

This is different from FE1.0. Adding `!!` will then surface the error
that `operator` modifier needs to be added (with corresponding fix).

idea/testData/quickfix/addExclExclCall/iterableWithBadIterator.fir.kt

@@ -0,0 +1,10 @@
+// "Add non-null asserted (!!) call" "true"
+
+class Some {
+    fun iterator(): Iterator<Int> = null!!
+}
+
+fun foo() {
+    val test: Some? = Some()
+    for (i in <caret>test) { }
+}

idea/testData/quickfix/addExclExclCall/iterableWithBadIterator.fir.kt.after

@@ -0,0 +1,10 @@
+// "Add non-null asserted (!!) call" "true"
+
+class Some {
+    fun iterator(): Iterator<Int> = null!!
+}
+
+fun foo() {
+    val test: Some? = Some()
+    for (i in test!!) { }
+}