Use yaml2obj for llvm-cov tests to see covmap

[chapuni]

Make llvm/test/tools/llvm-cov use covmap binary files generated from yaml2obj.

Some old tests (branch-*) are transformed and decoupled to source files and test scripts.

A small Makefile is included as a convenience.

• llvm-objcopy extracts essential sections, __llvm_prf_names, __llvm_covmap, and __llvm_covfun. I guess more fields might be reduced but I don't want to add extra manual steps. Let me know if someone knows further reduction.
• obj2yaml dumps final YAML files. At the moment, covmap fields are blobs.

I'm working on #115707 (and others) to make YAML files easier to read and handle.

https://discourse.llvm.org/t/rfc-objectyaml-with-coverage-map-sections/82953

[chapuni]

I've pushed #116057 and #116058 as examples. Their format will be changed.