Handle case with FirField properly. Previously the code was returning
STABLE_VALUE in that case.
Note that in fact the changed 'when' branch expects only FirJavaField,
as FirFieldImpl should be handled by FirBackingFieldSymbol-check at
the beginning of the method. However, it is impossible to refer to
FirJavaField directly due to module visibility
Enumerate all cases more carefully via exhaustive 'when',
it's more readable and will prevent potential issues in future in case
more subclasses are added.
^KT-58279 Fixed
Dmitry Savvinov