diagnostics: tweak to line-insertion fix-it hints with line-numbering

diagnostics: tweak to line-insertion fix-it hints with line-numbering

This commit slightly tweaks line-insertion fix-it hints, so that
with line-numbering, rather than e.g.:

 99 |       x = a;
    |+      break;
110 |     case 'b':
    |     ^~~~~~~~

we fill the margin with "+":

 99 |       x = a;
+++ |+      break;
110 |     case 'b':
    |     ^~~~~~~~

to emphasize that this is a suggested new line, rather than the user's
source.

gcc/ChangeLog:
	* diagnostic-show-locus.c (layout::start_annotation_line): Add
	"margin_char" parameter, defaulting to space.  Use it in place
	of pp_space for the initial part of the margin.
	(layout::print_leading_fixits): Use '+' when filling the margin
	of line-insertion fix-it hints.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/diagnostic-test-show-locus-bw-line-numbers.c
	(test_fixit_insert_newline): Update expected output to show '+'
	characters in margin of line-insertion fix-it hint.

From-SVN: r263605