Rollup merge of #144663 - Zalathar:empty-span, r=petrochenkov

coverage: Re-land "Enlarge empty spans during MIR instrumentation"

This allows us to assume that coverage spans will only be discarded during codegen in very unusual situations.

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This seemingly-simple change has a rather messy history:
- rust-lang/rust#140847
- rust-lang/rust#141650
- rust-lang/rust#144298
- rust-lang/rust#144480

Since then, a number of related changes have landed that should make it reasonable to try again:
- rust-lang/rust#144530
- rust-lang/rust#144560
- rust-lang/rust#144616

In particular, we have multiple fixes/mitigations, and a confirmed regression test for the original bug that is not triggered by re-landing the changes in this PR.

Author: Zalathar