coverage: Replace boolean options with a `CoverageLevel` enum
After #123409, and some discussion at https://github.com/rust-lang/rust/issues/79649#issuecomment-2042093553 and #124120, it became clear to me that we should have a unified concept of “coverage level”, instead of having several separate boolean flags that aren't actually independent.
This PR therefore introduces a `CoverageLevel` enum, to replace the existing boolean flags for `branch` and `mcdc`.
The `no-branch` value (for `-Zcoverage-options`) has been renamed to `block`, instructing the compiler to only instrument for block coverage, with no branch coverage or MD/DC instrumentation.
`@rustbot` label +A-code-coverage
cc `@ZhuUx` `@Lambdaris` `@RenjiSann`
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