diagnostic paths: loosen coupling between path-printing and path_summary

diagnostic paths: loosen coupling between path-printing and path_summary

Doing this makes followup work to add HTML path-printing cleaner.

gcc/ChangeLog:
	* tree-diagnostic-path.cc (struct path_summary::event_range): Move
	out of path_summary to...
	(struct event_range): ...here.
	(class path_summary): Convert to...
	(struct path_summary): ...this.
	(path_summary::m_ranges): Drop "private".
	(path_summary::print): Convert to...
	(print_path_summary_as_text): ...this, passing in the path_summary
	explicitly.
	(default_tree_diagnostic_path_printer): Update for above change.
	(selftest::test_empty_path): Likewise.
	(selftest::test_intraprocedural_path): Likewise.
	(selftest::test_interprocedural_path_1): Likewise.
	(selftest::test_interprocedural_path_2): Likewise.
	(selftest::test_recursion): Likewise.