[llvm] [llvm-cov][CoverageMapping] Fix LineCoverageStats incorrectly using gap region count (PR #203740)

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

@makslevental Thanks for the contribution.
I haven't inspected yet though, let me request a few things.

* Avoid blobs. I guess [yaml.makefile](https://github.com/llvm/llvm-project/blob/main/llvm/test/tools/llvm-cov/Inputs/yaml.makefile) would be informative.
  * Use `.proftext` instead of `.profdata`.
  * Avoid `.covmapping`. I suggest obj2yaml is acceptable for covmap sections. I'll remove `.covmapping` in the future.
* Could you find and pick up relevant issues? I guess a few issues would be relevant.

https://github.com/llvm/llvm-project/pull/203740