diagnostics: fixes to SARIF output [PR109360]

When adding validation of .sarif files against the schema
(PR testsuite/109360) I discovered various issues where we were
generating invalid .sarif files.

Specifically, in
  c-c++-common/diagnostic-format-sarif-file-bad-utf8-pr109098-1.c
the relatedLocations for the "note" diagnostics were missing column
numbers, leading to validation failure due to non-unique elements,
such as multiple:
	"message": {"text": "invalid UTF-8 character <bf>"}},
on line 25 with no column information.

Root cause is that for some diagnostics in libcpp we have a location_t
representing the line as a whole, setting a column_override on the
rich_location (since the line hasn't been fully read yet).  We were
handling this column override for plain text output, but not for .sarif
output.

Similarly, in diagnostic-format-sarif-file-pr111700.c there is a warning
emitted on "line 0" of the file, whereas SARIF requires line numbers to
be positive.

We also use column == 0 internally to mean "the line as a whole",
whereas SARIF required column numbers to be positive.

This patch fixes these various issues.

gcc/ChangeLog:
	PR testsuite/109360
	* diagnostic-format-sarif.cc
	(sarif_builder::make_location_object): Pass any column override
	from rich_loc to maybe_make_physical_location_object.
	(sarif_builder::maybe_make_physical_location_object): Add
	"column_override" param and pass it to maybe_make_region_object.
	(sarif_builder::maybe_make_region_object): Add "column_override"
	param and use it when the location has 0 for a column.  Don't
	add "startLine", "startColumn", "endLine", or "endColumn" if
	the values aren't positive.
	(sarif_builder::maybe_make_region_object_for_context): Don't
	add "startLine" or "endLine" if the values aren't positive.

libcpp/ChangeLog:
	PR testsuite/109360
	* include/rich-location.h (rich_location::get_column_override):
	New accessor.

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