Rollup merge of #127352 - Zalathar:coverage-info, r=oli-obk

coverage: Rename `mir::coverage::BranchInfo` to `CoverageInfoHi`

This opens the door to collecting and storing coverage information that is unrelated to branch coverage or MC/DC, during MIR building.

There is no change to the output of coverage instrumentation, but one deliberate change is that functions now *always* have an attached `CoverageInfoHi` (if coverage is enabled and they are eligible), even if they didn't collect any interesting branch information.

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`@rustbot` label +A-code-coverage

Author: GuillaumeGomez