[gdb/ada] Fix assert in ada_is_unconstrained_packed_array_type

[gdb/ada] Fix assert in ada_is_unconstrained_packed_array_type

On openSUSE Leap 42.3, with system compiler gcc 4.8.5 I run into:
...
(gdb) print u_one_two_three^M
src/gdb/gdbtypes.h:1050: internal-error: field: \
 Assertion `idx >= 0 && idx < num_fields ()' failed.^M
...

We run into trouble while doing this in
ada_is_unconstrained_packed_array_type:
...
1953          return TYPE_FIELD_BITSIZE (type, 0) > 0;
...
which tries to get field 0 from a type without fields:
...
(gdb) p type->num_fields ()
$6 = 0
...
which is the case because the type is a typedef:
...
(gdb) p type->code ()
$7 = TYPE_CODE_TYPEDEF
...

Fix this by using the type referenced by the typedef instead.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28323