diagnostics: don't generate URLs that won't be used

diagnostics: don't generate URLs that won't be used

The two places in diagnostics.c where URLs are printed both do
non-trivial work to generate the URL strings (including malloc/free), so
don't do this work if URL-printing is disabled.

gcc/ChangeLog:
	* diagnostic.c (print_any_cwe): Don't call get_cwe_url if URLs
	won't be printed.
	(print_option_information): Don't call get_option_url if URLs
	won't be printed.