diagnostics: add diagnostic_context::get_location_text

diagnostics: add diagnostic_context::get_location_text

No functional change intended.

gcc/ChangeLog:
	* diagnostic.cc (diagnostic_get_location_text): Convert to...
	(diagnostic_context::get_location_text): ...this, and convert
	return type from char * to label_text.
	(diagnostic_build_prefix): Update for above change.
	(default_diagnostic_start_span_fn): Likewise.
	(selftest::assert_location_text): Likewise.
	* diagnostic.h (diagnostic_context::get_location_text): New decl.

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