diagnostics: require callers of diagnostic_show_locus to be explicit about the printer [PR116613]

As work towards supporting multiple diagnostic outputs (where each
output has its own pretty_printer), update diagnostic_show_locus
so that the pretty_printer must always be explicitly passed in.

No functional change intended.

gcc/c-family/ChangeLog:
	PR other/116613
	* c-format.cc (selftest::test_type_mismatch_range_labels):
	Explicitly pass in dc.m_printer to diagnostic_show_locus.

gcc/ChangeLog:
	PR other/116613
	* diagnostic-show-locus.cc (diagnostic_context::maybe_show_locus):
	Convert param "pp" from * to &.  Drop logic for using the
	context's m_printer when the param is null.
	* diagnostic.h (diagnostic_context::maybe_show_locus): Convert
	param "pp" from * to &.
	(diagnostic_show_locus): Drop default "nullptr" value for pp
	param.  Assert that it and context are nonnull.  Pass pp by
	reference to maybe_show_locus.

gcc/testsuite/ChangeLog:
	PR other/116613
	* gcc.dg/plugin/expensive_selftests_plugin.c (test_richloc):
	Explicitly pass in dc.m_printer to diagnostic_show_locus.

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