FIR: fix label stealling and crash with multiple labels

Consider the code below

```
fun test() {
  a@ b@ {
    {}
  }
}
```

Currently when the code is converted to FIR, label `b` is bound to the
outer lambda and `a` gets bound to the inner lambda because it's not
consumed. This is wrong and also leads transfromation to fail with
exceptions because of the unexpected consumption of `a`.

This change fixes the above issue by designating a specific node in the
AST as the allowed user of a label when the label is added.

compiler/testData/diagnostics/tests/controlStructures/notAFunctionLabel_after.fir.kt

@@ -51,12 +51,21 @@ fun testLoopLabelInReturn(xs: List<Int>) {
 }
 
 fun testValLabelInReturn() {
-    L@ val fn = { return@L }
+    L@ val fn = { <!NOT_A_FUNCTION_LABEL!>return@L<!> }
     fn()
 }
 
 fun testHighOrderFunctionCallLabelInReturn() {
     L@ run {
-        return@L
+        <!NOT_A_FUNCTION_LABEL!>return@L<!>
+    }
+}
+
+fun testMultipleLabelsWithNestedLambda() {
+    l1@ l2@{
+        {
+            <!NOT_A_FUNCTION_LABEL!>return@l1<!>
+        }
+        return@l2
     }
 }