coverage: Store coverage source regions as `Span` until codegen
Historically, coverage spans were converted into line/column coordinates during the MIR instrumentation pass.
This PR moves that conversion step into codegen, so that coverage spans spend most of their time stored as `Span` instead.
In addition to being conceptually nicer, this also reduces the size of coverage mappings in MIR, because `Span` is smaller than 4x u32.
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There should be no changes to coverage output.
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