Do not change type in get_discrete_low_bound

Do not change type in get_discrete_low_bound

get_discrete_low_bound has this code:

    /* Set unsigned indicator if warranted.  */
    if (low >= 0)
      type->set_is_unsigned (true);

It's bad to modify a type in a getter like this, so this patch removes
this code.  FWIW I looked and this code has been there since at least
1999 (it was in the initial sourceware import).

Types in general would benefit from const-ification, which would
probably reveal more code like this, but I haven't attempted that.

Regression tested on x86-64 Fedora 36.

Reviewed-by: Kevin Buettner <kevinb@redhat.com>