Normalize Ada ptype to use a single "?"

Normalize Ada ptype to use a single "?"

Sometimes -- notably with unchecked unions -- the Ada "ptype" code
will print a "?" or "??" to indicate something unknown.  The choice of
what was printed was somewhat arbitrary, and in one case, Ada would
print an empty string rather than "?".

This patch normalizes the Ada code to use "?" rather than an empty
string or "??".  My reasoning here is that a single question mark is
enough to convey unknown-ness.

gdb/ChangeLog
2019-12-10  Tom Tromey  <tromey@adacore.com>

	* ada-typeprint.c (print_choices): Use a single "?".
	(print_variant_part): Print "?" if the discriminant name
	is not known.

gdb/testsuite/ChangeLog
2019-12-10  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/unchecked_union.exp: New file.
	* gdb.ada/unchecked_union/pck.adb: New file.
	* gdb.ada/unchecked_union/pck.ads: New file.
	* gdb.ada/unchecked_union/unchecked_union.adb: New file.
	* gdb-utils.exp (string_to_regexp): Also quote "?".

Change-Id: I3403040780a155ffa2c44c8e6a04ba86bc810e29