[Ada] cannot print slice of array defined as typedef

[Joel Brobecker <brobecker at adacore dot com>]

Hello,

A change we are making in the compiler to help preserve useful
types when using -feliminate-unused-debug-types exposed a small
hole in our value-printing code.

One example of the problem happens when trying to print a slice
of an array pointer.  If the variable is defined as a pointer to
the typedef of an array, then we fail to print the slice, like so:

    (gdb) p arr_ptr(1..2)
    cannot take slice of non-array

I would normally submit fix + testsuite update in one patch, except
that in this case, Eric Botcazou is the author of the patch, whereas
I'm the one who took care of the testsuite update.

I will commit in a couple of days...