d: Remove dependency on front-end OutBuffer for diagnostic and deps file generation.

d: Remove dependency on front-end OutBuffer for diagnostic and deps file generation.

gcc/d/ChangeLog:

	* d-diagnostic.cc (expand_d_format): Replace OutBuffer with obstack.
	(d_diagnostic_report_diagnostic): Don't free xformat.
	* d-lang.cc (struct d_option_data): Change deps_target type from
	OutBuffer to vec <const char *>.
	(deps_add_target): Update to push each target to deps_target.
	(deps_write_string): New function.
	(deps_write): Change buffer type to obstack* and remove colmax.
	(d_init_options): Update initialization of deps_target.
	(d_parse_file): Replace OutBuffer with obstack.