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.
(e.g., `nullable.a = b`), and use positioning strategies to locate the
dot in the LHS expression.
Without it, only the callee reference is reported on, which makes the
highlighting of the error and application of quickfixes incorrect in the
IDE.
Also fixed issue with annotated and/or labeled expressions on LHS of
assignment (e.g., `(@Ann label@ i) = 34`).
Currently VAL_REASSIGNMENT are only reported on direct assignments.
Reassignments in the form of, for example, `+=` are reported as
`VARIABLE_EXPECTED`, which differs from FE1.0.
Mikhail Glukhikh