pretty-print: support URL escape sequences (PR 87488)

pretty-print: support URL escape sequences (PR 87488)

https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda
describes an emerging standard for embedding URLs in escape sequences
for marking up text output.  This is supported e.g. by recent releases
of GNOME Terminal.

This patch adds support to our pretty-printing framework for emitting
URLs.

A followup patch uses this to add URLs to the pertinent documentation
for the output of -fdiagnostics-show-option.

gcc/ChangeLog:
	PR 87488
	* common.opt (fdiagnostics-urls=): New option.
	(diagnostic-url.h): Add SourceInclude.
	(diagnostic_url_rule): New enum.
	* diagnostic-color.c: Include "diagnostic-url.h".
	(diagnostic_urls_enabled_p): New function.
	* diagnostic-url.h: New file.
	* diagnostic.c: Include "diagnostic-url.h".
	(diagnostic_urls_init): New function.
	* diagnostic.h (diagnostic_urls_init): New decl.
	* doc/invoke.texi (Diagnostic Message Formatting Options): Add
	-fdiagnostics-urls to the list.
	(-fdiagnostics-urls): New option.
	* gcc.c (driver_handle_option): Handle OPT_fdiagnostics_urls_.
	(driver::global_initializations): Call diagnostic_urls_init.
	* opts-global.c (init_options_once): Likewise.
	* opts.c (common_handle_option): Handle OPT_fdiagnostics_urls_.
	* pretty-print.c (pretty_printer::pretty_printer): Initialize
	show_urls.
	(pp_begin_url): New function.
	(pp_end_url): New function.
	(selftest::test_urls): New selftest.
	(selftest::pretty_print_c_tests): Call it.
	* pretty-print.h (pretty_printer::show_urls): New field.
	(pp_begin_url): New decl.
	(pp_end_url): New decl.

gcc/testsuite/ChangeLog:
	PR 87488
	* lib/prune.exp (TEST_ALWAYS_FLAGS): Add -fdiagnostics-urls=never.

From-SVN: r276841