Add $_ada_exception convenience variable

Add $_ada_exception convenience variable

This adds the $_ada_exception convenience variable.  It is set by the
Ada exception catchpoints, and holds the address of the exception
currently being thrown.  This is useful because it allows more
fine-grained filtering of exceptions than is possible using the
existing "catch" syntax.

This also simplifies Ada catchpoints somewhat; because the catchpoint
must now carry the "kind", it's possible to remove many helper
functions.

gdb/ChangeLog
2019-10-02  Tom Tromey  <tromey@adacore.com>

	* NEWS: Add $_ada_exception entry.
	* ada-lang.c (struct ada_catchpoint): Add constructor.
	<m_kind>: New member.
	(allocate_location_exception, re_set_exception): Remove
	"ex" parameter.
	(should_stop_exception): Compute $_ada_exception.
	(check_status_exception, print_it_exception)
	(print_one_exception, print_mention_exception): Remove
	"ex" parameter.
	(allocate_location_catch_exception, re_set_catch_exception)
	(check_status_exception, print_it_catch_exception)
	(print_one_catch_exception, print_mention_catch_exception)
	(print_recreate_catch_exception)
	(allocate_location_catch_exception_unhandled)
	(re_set_catch_exception_unhandled)
	(check_status_exception, print_it_catch_exception_unhandled)
	(print_one_catch_exception_unhandled)
	(print_mention_catch_exception_unhandled)
	(print_recreate_catch_exception_unhandled)
	(allocate_location_catch_assert, re_set_catch_assert)
	(check_status_assert, print_it_catch_assert)
	(print_one_catch_assert, print_mention_catch_assert)
	(print_recreate_catch_assert)
	(allocate_location_catch_handlers, re_set_catch_handlers)
	(check_status_handlers, print_it_catch_handlers)
	(print_one_catch_handlers, print_mention_catch_handlers)
	(print_recreate_catch_handlers): Remove.
	(create_ada_exception_catchpoint): Update.
	(initialize_ada_catchpoint_ops): Update.

gdb/doc/ChangeLog
2019-10-02  Tom Tromey  <tromey@adacore.com>

	* gdb.texinfo (Set Catchpoints, Convenience Vars): Document
	$_ada_exception.

gdb/testsuite/ChangeLog
2019-10-02  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/catch_ex_std.exp: Add $_ada_exception test.