[binutils-gdb] Remove cleanup from print_mention_exception

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 862d101ada6b6e8496e545c0bcd801cf8b9a46c1 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 862d101ada6b6e8496e545c0bcd801cf8b9a46c1

Remove cleanup from print_mention_exception

This removes a cleanup from print_mention_exception by using
string_printf.

ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	* ada-lang.c (print_mention_exception): Use std::string.