Come up with new --completion option.

Come up with new --completion option.

2018-06-28  Martin Liska  <mliska@suse.cz>

	* common.opt: Introduce -completion option.
	* gcc.c (driver_handle_option): Handle it.
	(driver::main): Print completions if completion
        is set.
	* opt-suggestions.c (option_proposer::get_completions):
        New function.
	(option_proposer::suggest_completion): Likewise.
	(option_proposer::find_param_completions): Likewise.
	(verify_autocompletions): Likewise.
	(test_completion_valid_options): Likewise.
	(test_completion_valid_params): Likewise.
	(in_completion_p): Likewise.
	(empty_completion_p): Likewise.
	(test_completion_partial_match): Likewise.
	(test_completion_garbage): Likewise.
	(opt_proposer_c_tests): Likewise.
	* opt-suggestions.h: Declare new functions.
	* opts.c (common_handle_option): Handle OPT__completion_.
	* selftest-run-tests.c (selftest::run_tests): Add
        opt_proposer_c_tests.
	* selftest.c (assert_str_startswith): New.
	* selftest.h (assert_str_startswith): Likewise.
	(opt_proposer_c_tests): New.
	(ASSERT_STR_STARTSWITH): Likewise.

From-SVN: r262210