Support prefixes in diagnostic_show_locus

Support prefixes in diagnostic_show_locus

Previously, diagnostic_show_locus saved and restored the pretty_printer's
prefix, clearing it for the duration of the call.

I have a patch kit in development that can benefit from applying a prefix
to the output of d_s_l, so this patch adds support to d_s_l for printing
such prefixes.

It moves the save and restore of the pp's prefix from d_s_l to all of its
callers, and updates diagnostic-show-locus.c to properly handle prefixes.

gcc/c-family/ChangeLog:
	* c-opts.c (c_diagnostic_finalizer): Temporarily clear prefix when
	calling diagnostic_show_locus, rather than destroying it afterwards.

gcc/ChangeLog:
	* diagnostic-show-locus.c (layout::print_gap_in_line_numbering):
	Call pp_emit_prefix.
	(layout::print_source_line): Likewise.
	(layout::start_annotation_line): Likewise.
	(diagnostic_show_locus): Remove call to temporarily clear the
	prefix.
	(selftest::test_one_liner_fixit_remove): Add test coverage for the
	interaction of pp_set_prefix with rulers and fix-it hints.
	* diagnostic.c (default_diagnostic_finalizer): Temporarily clear
	prefix when calling diagnostic_show_locus, rather than destroying
	it afterwards.
	(print_parseable_fixits): Temporarily clear prefix.
	* pretty-print.c (pp_format): Save and restore line_length, rather
	than assuming it is zero.
	(pp_output_formatted_text): Remove assertion that line_length is
	zero.

gcc/fortran/ChangeLog:
	* error.c (gfc_diagnostic_starter): Clear the prefix before
	calling diagnostic_show_locus.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/diagnostic_group_plugin.c (test_begin_group_cb):
	Clear the prefix before emitting the "END GROUP" line.
	* gcc.dg/plugin/diagnostic_plugin_test_show_locus.c
	(custom_diagnostic_finalizer): Temporarily clear prefix when
	calling diagnostic_show_locus, rather than destroying it
	afterwards.

From-SVN: r276433