diagnostics: make m_text_callbacks private

No functional change intended.

gcc/ChangeLog:
	* diagnostic-show-locus.cc (diagnostic_context::show_locus):
	Update for renaming of text callbacks fields.
	* diagnostic.cc (diagnostic_context::initialize): Likewise.
	* diagnostic.h (class diagnostic_context): Add "friend" for
	accessors to m_text_callbacks.
	(diagnostic_context::m_text_callbacks): Make private, and add an
	"m_" prefix to field names.
	(diagnostic_starter): Convert from macro to inline function.
	(diagnostic_start_span): New.
	(diagnostic_finalizer): Convert from macro to inline function.

gcc/fortran/ChangeLog:
	* error.cc (gfc_diagnostics_init): Use diagnostic_start_span.

gcc/ChangeLog:
	* selftest-diagnostic.cc
	(test_diagnostic_context::test_diagnostic_context): Use
	diagnostic_start_span.
	* tree-diagnostic-path.cc (struct event_range): Likewise.

gcc/testsuite:
	* gcc.dg/plugin/diagnostic_group_plugin.c: Use
	diagnostic_start_span.

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