Since many labels are not present in the FIR tree, this checker is
implemented as a syntax checker. Comparing with FE1.0, this change
reports some REDUNDANT_LABEL_WARNING that FE1.0 has missed, especially
LHS of assignments.
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.
vladislav.grechko