Rollup merge of #117651 - Zalathar:fold-sums, r=cjgillot

coverage: Simplify building coverage expressions based on sums

This is a combination of some interlinked changes to the code that creates coverage counters/expressions for nodes and edges in the coverage graph:

- Some preparatory cleanups in `MakeBcbCounters::make_branch_counters`
- Use `BcbCounter` (instead of `CovTerm`) when building coverage expressions
  - This makes it easier to introduce a fold for building sums
- Simplify the creation of coverage expressions based on sums, by having `Iterator::fold` do much of the work
- Get rid of the awkward `BcbBranch` enum, and replace it with graph edges represented as `(from_bcb, to_bcb)`
  - This further simplifies the body of the fold

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