Auto merge of #124603 - Zalathar:mcdc-mappings, r=Nadrieril

coverage: Split out MC/DC mappings from `BcbMappingKind`

These variants were added to `BcbMappingKind` as part of the [MC/DC coverage](https://en.wikipedia.org/wiki/Modified_Condition/Decision_Coverage) implementation in #123409, because that was the path-of-least-resistance for integrating them into the existing code.

However, they ultimately represent complex concepts that the enum was not intended to handle, leading to more complexity in the code that processes them. This PR therefore follows in the footsteps of #124545, and splits the MC/DC mappings out into their own dedicated vectors of structs.

After that, `BcbMappingKind` itself ends up having only one variant (`Code`), so this PR also flattens that enum into its enclosing struct, renamed to `mapping::CodeMapping`.

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No functional changes.

This will conflict slightly with #124571, but hopefully that should be easy to resolve either way.

`@rustbot` label +A-code-coverage