Add "can be builtin" setting to ProcDirective

[spookydonut]

Fixes #202

[SpaceManiac]

Do we need to afford special status to builtins here? Can we simply say these lint settings "must be on the declaration" or not?

[spookydonut]

Do we need to afford special status to builtins here? Can we simply say these lint settings "must be on the declaration" or not?

@SpaceManiac maybe just allow compatible lints on built-ins regardless?

[SpaceManiac]

@spookydonut why does "built-in-ness" matter at all? Are there lints that don't work if they go on overrides rather than /proc/ declarations?