[llvm] Add executed MC/DC TestVectors to `llvm-cov export` (PR #105511)

[Alan Phipps via llvm-commits]

evodius96 wrote:

Sorry for the long delay in reviewing this code. Thank you for your contribution to improving this.  In general, LGTM. However, can you also add a test to verify the JSON output?  You can extend one of the existing test files.  Also, it would be good to do a rebase given the age of the PR.  Thanks!

https://github.com/llvm/llvm-project/pull/105511