coverage: Skip spans that can't be un-expanded back to the function body
When we extract coverage spans from MIR, we try to "un-expand" them back to spans that are inside the function's body span.
In cases where that doesn't succeed, the current code just swaps in the entire body span instead. But that tends to result in coverage spans that are completely unrelated to the control flow of the affected code, so it's better to just discard those spans.
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Extracted from #118305, since this is a general improvement that isn't specific to branch coverage.
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`@rustbot` label +A-code-coverage
80c94e81d9