lint: give malformed linter directives the "compile" check
lint: give malformed linter directives the "compile" check

The "compile" check is our catch-all for problems affecting the
analysis itself: type errors, invalid configuration, and so on. We
forgot to give malformed linter directives the "compile" check.

This caused them to never be shown, because "" isn't on the list of
allowed checks.

Fixes gh-515

dominikh/go-tools,dominikh/go-tools,dominikh/go-tools,dominikh/go-tools