c-format.cc: add ctors to format_check_results and format_check_context

c-format.cc: add ctors to format_check_results and format_check_context

This is a minor cleanup I spotted whilst working on another patch.
No functional change intended.

gcc/c-family/ChangeLog:
	* c-format.cc (format_check_results::format_check_results): New
	ctor.
	(struct format_check_context): Add ctor; add "m_" prefix to all
	fields.
	(check_format_info): Use above ctors.
	(check_format_arg): Update for "m_" prefix to
	format_check_context.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>