Auto merge of #121026 - Zalathar:version, r=oli-obk

coverage: Correctly report and check LLVM's coverage mapping version

I was puzzled by the fact that the LLVM 18 update (#120055) didn't need to modify this version check, despite the fact that LLVM 18 uses a newer version of the coverage mapping format.

This turned out to be because we were inappropriately hard-coding a specific version (`Version6`) in the C++ wrapper, instead of using `CovMapVersion::CurrentVersion` to reflect the version actually used by LLVM on our behalf.

This PR fixes that, and also changes the Rust-side version check to accept the new coverage mapping version used by LLVM 18, since the necessary compatibility work has already been done.

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### Quick history of `LLVMRustCoverageMappingVersion`:

- Originally it returned LLVM's `coverage::CovMapVersion::CurrentVersion`, as intended. The Rust-side code would verify it, and also embed it as the actual coverage version number in the output binary.
- At some point it was changed to a hard-coded value, to work around a (now-irrelevant) compatibility issue. This was incorrect (but mostly benign), because the override should have been performed on the Rust side instead, after verifying LLVM's value.
- Later contributors dutifully updated the hard-coded value, because they didn't have enough context to identify the problem.
- With this PR, it once again returns LLVM's current coverage version number, and the Rust-side code checks it against an expected range. We don't override the result, but we do indicate where that override should occur if it ever becomes necessary.

Author: bors