Store warnings in externs? #3198

Maybe this was discussed before, I can't remember. Just thinking it'd be nice to have, since then warnings could always be shown regardless of whether the build is incremental or not.

I know -psa has --stash to implement this on top of the compiler interface, but it's not completely reliable, so by storing in the externs we'd only have one set of cache invalidation related bugs to deal with. 😉