new diagnostic for meaningless lint directive

[a-frantz]

Lint directives in the preamble should not fire this diagnostic. This is to support the "boilerplate" use case where someone just wants to blanket except certain rules for any WDL document.

Other lint directives that don't actually suppress anything should emit a note. Detecting this will involve tracking what rules are being called with exceptable_add() and then looking through the CST.