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

[Fabian Meumertzheim via llvm-commits]

fmeum 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.

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