Allow a pretty-printer without a to_string method

Allow a pretty-printer without a to_string method

PR python/16047 points out that, while the documentation says that the
to_string method is optional for a pretty-printer, the code disagrees
and throws an exception.  This patch fixes the problem.  varobj is
already ok here.

Tested on x86-64 Fedora 26.

gdb/ChangeLog
2018-09-08  Tom Tromey  <tom@tromey.com>

	PR python/16047:
	* python/py-prettyprint.c (pretty_print_one_value): Check for
	to_string method.

gdb/testsuite/ChangeLog
2018-09-08  Tom Tromey  <tom@tromey.com>

	PR python/16047:
	* gdb.python/py-prettyprint.py (pp_int_typedef3): New class.
	(register_pretty_printers): Register new printer.
	* gdb.python/py-prettyprint.exp (run_lang_tests): Add int_type3
	test.
	* gdb.python/py-prettyprint.c (int_type3): New typedef.
	(an_int_type3): New global.