When debugging diagnostic-show-locus.c, it's invaluable to have a
"ruler" showing column numbers.

This patch adds in support via a new "show_ruler_p" flag within
the diagnostic_context.  There's no direct way for end-users to enable
this, but plugins can enable it by setting the flag, so the
plugin that tests the diagnostic subsystem uses this to verify that
the ruler is correctly printed.

gcc/ChangeLog:
	* diagnostic-show-locus.c (layout::layout): Call show_ruler
	if show_ruler_p was set on the context.
	(layout::show_ruler): New method.
	* diagnostic.h (struct diagnostic_context): Add field
	"show_ruler_p".

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/diagnostic-test-show-locus-bw.c
	(test_very_wide_line): Add ruler to expected output.
	* gcc.dg/plugin/diagnostic-test-show-locus-color.c
	(test_very_wide_line): Likewise.
	* gcc.dg/plugin/diagnostic_plugin_test_show_locus.c
	(test_show_locus): Within the handling of "test_very_wide_line",
	enable show_ruler_p on the diagnostic context.

From-SVN: r236080