Q about Ada and value ranges in types

[Richard Kenner]

    RTH has been suggesting to use build_int_cst (etype, 0) instead.

Indeed.  I was trying to minimize the change, but such cleanups are always
useful.  This was also missing a protection on INTEGER_TYPE_P.  I just got
a good bootstrap of Ada on x86_64 with this and a patch from Diego to fix
the other problem and also remove the kludge that's fixed by this.