[llvm] [llvm-profdata] Reject merging single-byte-coverage with count profiles (PR #213177)

[Ellis Hoag via llvm-commits]

https://github.com/ellishg approved this pull request.

LGTM. I have a slight preference to using `instrprof_error::unsupported_version` for simplicity and consistency, but it's up to you

https://github.com/llvm/llvm-project/pull/213177