Tweak Interface_field_reference_expression::do_get_backend to apply an
    additional backend type conversion to the returned result.  This is
    needed due to the fact that the top level type of the expression is a
    function descriptor, however the value being manufactured is a pointer
    to <thunk-descriptor, value> struct.
    
    Reviewed-on: https://go-review.googlesource.com/51712

From-SVN: r250686