Rollup merge of #134497 - Zalathar:spans, r=jieyouxu

coverage: Store coverage source regions as `Span` until codegen (take 2)

This is an attempt to re-land #133418:

> 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.

That PR was reverted by #133608, because in some circumstances not covered by our test suite we were emitting coverage metadata that was causing `llvm-cov` to exit with an error (#133606).

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The implementation here is *mostly* the same, but adapted for subsequent changes in the relevant code (e.g. #134163).

I believe that the changes in #134163 should be sufficient to prevent the problem that required the original PR to be reverted. But I haven't been able to reproduce the original breakage in a regression test, and the `llvm-cov` error message is extremely unhelpful, so I can't completely rule out the possibility of this breaking again.

r? jieyouxu (reviewer of the original PR)

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