Fix debug info for packed array types in Ada

Fix debug info for packed array types in Ada

Packed array types are sometimes represented with integer types under the
hood in Ada, but we nevertheless need to emit them as array types in the
debug info so we have the types.get_array_descr_info langhook for this
purpose; but it is not invoked from modified_type_die, which causes:

FAIL: gdb.ada/arrayptr.exp: scenario=minimal: print pa_ptr.all
FAIL: gdb.ada/arrayptr.exp: scenario=minimal: print pa_ptr.all(3)

in the GDB testsuite.

gcc/
	* dwarf2out.c (modified_type_die): Deal with all array types earlier
	and use local variable consistently throughout the function.