coverage: Treat `#[automatically_derived]` as `#[coverage(off)]`
One of the contributing factors behind https://github.com/rust-lang/rust/issues/141577#issuecomment-3120667286 was the presence of derive-macro-generated code containing nested closures.
Coverage instrumentation already has a heuristic for skipping code marked with `#[automatically_derived]` (rust-lang/rust#120185), because derived code is usually not worth instrumenting, and also has a tendency to trigger vexing edge-case bugs in coverage instrumentation or coverage codegen.
However, the existing heuristic only applied to the associated items directly within an auto-derived impl block, and had no effect on closures or nested items within those associated items.
This PR therefore extends the search for `#[coverage(..)]` attributes to also treat `#[automatically_derived]` as an implied `#[coverage(off)]` for the purposes of coverage instrumentation.
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This change doesn’t rule out an entire category of bugs, because it only affects code that actually uses the auto-derived attribute. But it should reduce the overall chance of edge-case macro span bugs being observed in the wild.
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