Reject slicing a packed array

In Ada mode, gdb rejects slicing a packed array.  However, with
-fgnat-encodings=minimal, gdb will instead print incorrect results.
This patch changes gdb to also reject slicing a packed array in this
mode.

FWIW I believe that this rejection is a gdb limitation.  Removing it
looked complicated, though, and meanwhile my main goal for the time
being is to bring the DWARF encodings up to par with Gnat encodings.

gdb/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_is_any_packed_array_type): New function.
	(ada_evaluate_subexp) <case TERNOP_SLICE>: Use it.

gdb/testsuite/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/mod_from_name.exp: Test printing slice.