This patch adds a minimum width to the left margin used for printing
line numbers.   I set the default to 6.  Hence rather than:

some-filename:9:1: some message
9 | some source text
  | ^~~~~~~~~~~~~~~~
some-filename:10:1: another message
10 | more source text
   | ^~~~~~~~~~~~~~~~

we now print:

some-filename:9:42: some message
    9 | some source text
      | ^~~~~~~~~~~~~~~~
some-filename:10:42: another message
   10 | more source text
      | ^~~~~~~~~~~~~~~~

This implicitly fixes issues with margins failing to line up due
to different lengths of the number when we haven't read the full
file yet and so don't know the highest possible line number, for
line numbers up to 99999.

Doing so adds some whitespace on the left-hand side, for non-huge
files, at least.  I believe that this makes it easier to see where each
diagnostic starts, by visually breaking things up at the leftmost
column; my hope is to make it easier for the eye to see the different
diagnostics as if they were different "paragraphs".

gcc/ChangeLog:
	* common.opt (fdiagnostics-minimum-margin-width=): New option.
	* diagnostic-show-locus.c (layout::layout): Apply the minimum
	margin width.
	(layout::start_annotation_line): Only print up to 3 of the
	margin character, to avoid touching the left-hand side.
	(selftest::test_diagnostic_show_locus_fixit_lines): Update for
	minimum margin width, as set by test_diagnostic_context's ctor.
	(selftest::test_fixit_insert_containing_newline): Likewise.
	(selftest::test_fixit_insert_containing_newline_2): Likewise.
	(selftest::test_line_numbers_multiline_range): Clear
	dc.min_margin_width.
	* diagnostic.c (diagnostic_initialize): Initialize
	min_margin_width.
	* diagnostic.h (struct diagnostic_context): Add field
	"min_margin_width".
	* doc/invoke.texi: Add -fdiagnostics-minimum-margin-width=.
	* opts.c (common_handle_option): Handle
	OPT_fdiagnostics_minimum_margin_width_.
	* selftest-diagnostic.c
	(selftest::test_diagnostic_context::test_diagnostic_context):
	Initialize min_margin_width to 6.
	* toplev.c (general_init): Initialize global_dc->min_margin_width.

gcc/testsuite/ChangeLog:
	* gcc.dg/missing-header-fixit-3.c: Update expected indentation
	to reflect minimum margin width.
	* gcc.dg/missing-header-fixit-4.c: Likewise.
	* gcc.dg/plugin/diagnostic-test-show-locus-bw-line-numbers.c:
	Likewise.
	* gcc.dg/plugin/diagnostic-test-show-locus-color-line-numbers.c:
	Likewise.
	* gcc.dg/plugin/diagnostic-test-show-locus-bw-line-numbers-2.c:
	New test.
	* gcc.dg/plugin/plugin.exp (plugin_test_list): Add it.

From-SVN: r265178