Add support for alias `a AT b` initializers targeting local members

[mhasel]

Currently, reference initializers are only supported for global target variables (and stack-local members, i.e. functions and VAR_TEMP). It would be nice to extend the alias functionality to work with local references in stateful POUs also, e.g.:

FUNCTION_BLOCK foo
    VAR
        foo AT bar : DINT;
        bar : DINT;
    END_VAR
END_FUNCTION_BLOCK