diagnostics: use refs rather than pointers for diagnostic_{path,context}

diagnostics: use refs rather than pointers for diagnostic_{path,context}

Use const & rather than const * in various places where it can't be null
and can't change.

No functional change intended.

gcc/ChangeLog:
	* diagnostic-path.cc: Replace "const diagnostic_path *" with
	"const diagnostic_path &" throughout, and "diagnostic_context *"
	with "diagnostic context &".
	* diagnostic.cc (diagnostic_context::show_any_path): Pass
	reference in call to print_path.
	* diagnostic.h (diagnostic_context::print_path): Convert param
	to a reference.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>