gimple-ssa-warn-access.cc: add missing auto_diagnostic_group

gimple-ssa-warn-access.cc: add missing auto_diagnostic_group

Whilst working on SARIF output I noticed some places where followup notes
weren't being properly associated with their warnings in
gcc/gimple-ssa-warn-access.cc.

Fixed thusly.

gcc/ChangeLog:
	* gimple-ssa-warn-access.cc (warn_string_no_nul): Add
	auto_diagnostic_group to group any warning with its note.
	(maybe_warn_for_bound): Likewise.
	(check_access): Likewise.
	(warn_dealloc_offset): Likewise.
	(pass_waccess::maybe_warn_memmodel): Likewise.
	(pass_waccess::maybe_check_dealloc_call): Likewise.
	(pass_waccess::warn_invalid_pointer): Likewise.
	(pass_waccess::check_dangling_stores): Likewise.

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