Rollup merge of rust-lang#147170 - Zalathar:directive, r=jieyouxu

compiletest: Pass around `DirectiveLine` instead of bare strings

This is an incremental step towards being able to clean up and centralize compiletest directive parsing.

My original plan was to add features to `DirectiveLine`, and then gradually migrate parsing code to use those features. However, that turned out to be impractical, because of how the existing directive parsers call each other. So instead this PR focuses on getting them to all take `DirectiveLine` instead of bare strings, to enable incremental work in the future.

Because this is part of an ongoing cleanup, I've prioritised clean diffs over nice code, because much of this code is going to be modified again when `DirectiveLine` is more capable.

r? jieyouxu

Author: Zalathar