Rollup merge of #121233 - Zalathar:extra-directives, r=oli-obk

Move the extra directives for `Mode::CoverageRun` into `iter_header`

When these extra directives were ported over as part of #112300, it made sense to introduce `iter_header_extra` and pass them in as an extra argument.

But now that #120881 has added a `mode` parameter to `iter_header` for its own purposes, it's slightly simpler to move the coverage special-case code directly into `iter_header` as well. This lets us get rid of `iter_header_extra`.