[llvm] [llvm-cov] Migrate gcov to OptTable (PR #224955)

[Jordan Rupprecht via llvm-commits]

rupprecht wrote:

> Backward compatibility is minimal concern

Yeah, but on the other hand, Hyrum's law exists :)

I spot checked usage and couldn't find any usage of `-gcno` / `-gcda` with just single dashes, outside of these unit tests. If I missed something, hopefully it'll be something we can fix at the invocation instead of adding extra flag compatibility handling here.

https://github.com/llvm/llvm-project/pull/224955