[llvm] [llvm-cov] Add support for baseline coverage (PR #117910)

[via llvm-commits]

gulfemsavrun wrote:

> Notes for reviewers:
> 
> * I could gate the new behavior behind a new flag (e.g. `-empty-profile`) so that forgetting to pass `-instr-profile` without the intention to get a baseline coverage report would still be an error.
> * Instead of passing an optional reader everywhere, I could try to extract a more narrowly scoped interface for coverage data out of `IndexedInstrProfileReader` and provide an implementation that returns all zero counters. Let me know if you would prefer that approach and the more drastic API changes to `CoverageMapping` this would require are acceptable.

Overall, it looks good to me. But, please put the new behavior behind a flag so that we do not change the default behavior.

https://github.com/llvm/llvm-project/pull/117910