Cautiously give PC more control over docComments table

This is needed after https://github.com/scala/scala/pull/2208 changed the role of `DocComments` 's `docComments` table, by removing `DocDef`s from the AST after even the `ScaladocAnalyzer`'s runs, making the PC utterly dependent on said table.

This gives the necessary trait to relax its cleaning policy in new `Typer` runs.

Review by @dragos, @retronym.