Adam Gundry authored Jan 10, 2023 and Marge Bot committed Jan 18, 2023

This adds `-Werror=<group>` and `-fwarn-<group>` flags for warning
groups as well as individual warnings. Previously these were defined
on an ad hoc basis so for example we had `-Werror=compat` but not
`-Werror=unused-binds`, whereas we had `-fwarn-unused-binds` but not
`-fwarn-compat`. Fixes #22182.