This brings this control in line with other diagnostic controls we have which operate on a per-group level.
'DefaultIgnoreWarnings' diagnostic group option applies to all warnings belonging to a certain diagnostic group.
The inheritance rules are:
- Marking a diagnostic group as 'DefaultIgnoreWarnings' means warnings belonging to this group will not be emitted by-default
- Warnings belonging to sub-groups of this group will also not be emitted by-default
- Enabling a 'DefaultIgnoreWarnings' group (with '-Werror','-Wwarning', etc.) means warnings belonging to this group will be emitted.
- Warnings belonging to sub-groups of this group will also be emitted.
- Warnings belonging to super-groups of this group will not be affected.
