Rollup merge of #127157 - Zalathar:unexpand, r=cjgillot

coverage: Avoid getting extra unexpansion info when we don't need it

Several callers of `unexpand_into_body_span_with_visible_macro` would immediately discard the additional macro-related information, which is wasteful. We can avoid this by having them instead call a simpler method that just returns the span they care about.

This PR also moves the relevant functions out of `coverage::spans::from_mir` and into a new submodule `coverage::unexpand`, so that calling them from `coverage::mappings` is less awkward.

There should be no actual changes to coverage-instrumentation output, as demonstrated by the absence of test updates.

Author: matthiaskrgr