Disable -Wformat-nonliteral in parts of printcmd.c

Disable -Wformat-nonliteral in parts of printcmd.c

commit 3322c5d9a1 ("Remove unneeded explicit .o targets") broke the
build with clang, because -Wno-format-nonliteral was in fact needed.
This patch fixes the problem by introducing
DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL and using it in printcmd.c.  This
seems preferable to reverting the patch because now the warning
suppression is more targeted.

gdb/ChangeLog
2018-09-05  Simon Marchi  <simon.marchi@ericsson.com>

	* printcmd.c (printf_c_string): Use
	DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL.
	(printf_wide_c_string, printf_pointer, ui_printf): Likewise.

include/ChangeLog
2018-09-05  Simon Marchi  <simon.marchi@ericsson.com>

	* diagnostics.h (DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL): New macro.