Print non-Ada unions without crashing

Print non-Ada unions without crashing

ada-lang.c is a bit too eager trying to decode unions in the Ada style
-- looking for discriminants and such.  This causes crashes when
printing a non-Ada union in Ada mode, something that can easily happen
when printing a value from history or certain registers on AArch64.

This patch fixes the bug by changing ada-lang.c to only apply special
Ada treatment to types coming from an Ada CU.  This in turn required a
couple of surprising changes.

First, some of the Ada code was already using HAVE_GNAT_AUX_INFO to
decide whether a type had already been fixed -- such types had
INIT_CPLUS_SPECIFIC called on them.  This patch changes these spots to
use the "none" identifier instead.

This then required changing value_rtti_type to avoid changing the
language-specific object attached to an Ada type, which seems like a
good change regardless.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-04-19  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_is_variant_part, ada_to_fixed_type_1):
	Check ADA_TYPE_P.
	(empty_record, ada_template_to_fixed_record_type_1)
	(template_to_static_fixed_type)
	(to_record_with_fixed_variant_part): Use INIT_NONE_SPECIFIC.
	* cp-abi.c (value_rtti_type): Check HAVE_CPLUS_STRUCT.
	* gdbtypes.h (INIT_NONE_SPECIFIC, ADA_TYPE_P): New
	macros.

gdb/testsuite/ChangeLog
2019-04-19  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/ptype_union.c: New file.
	* gdb.ada/ptype_union.exp: New file.