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/nestedLoopsWithMultipleLabels.kt

@@ -0,0 +1,9 @@
+// FIR_IDENTICAL
+fun test() {
+    a@ b@ while(true) {
+        val f = {
+            <!NOT_A_FUNCTION_LABEL!>return@a<!>
+        }
+        break@b
+    }
+}