Handling of arrays with optimized-out bounds

In Ada, sometimes the compiler must emit array bounds by referencing
an artificial variable that's created for this purpose.  However, with
optimization enabled, these variables can be optimized away.

Currently this can result in displays like:

    (gdb) print mumble
    $1 = (warning: unable to get bounds of array, assuming null array
    )

This patch changes this to report that the array is optimized-out,
instead, which is closer to the truth, and more generally useful.  For
example, Python pretty-printers can now recognize this situation.

In order to accomplish this, I introduced a new PROP_OPTIMIZED_OUT
enumerator and changed one place to use it.  Reusing the "unknown"
state wouldn't work properly, because in C it is normal for array
bounds to be unknown.