Fix range type signed-ness heuristic

Fix range type signed-ness heuristic

The code to create a range type has a heuristic to decide whether the
range is unsigned.  However, this heuristic can fail if the upper
bound of the range has its high bit set, because the test is done
using LONGEST.

With this patch, if the underlying type of a range is unsigned, then
the range will always be unsigned.  A new test is included.

Regression tested on x86-64 Fedora 34.  We've also been using this
internally at AdaCore for a while.