Add support for llvm-cov covered regions.

[rkjnsn]

Is your feature request related to a problem? Please describe.
When viewing test coverage in VS Code, I'd like to be able to see when a region of code within a line is not covered.

Describe the solution you'd like
I'm using llvm-cov, which supports generating region-based coverage in addition to function- and line-based coverage. This is useful to see when a part of a line is never executed, such as due to a short-circuiting operator or conditional expression.

llvm-cov has a native JSON output format, which includes this information. It would be great if Coverage Gutters could parse this format and use the region based information to show region execution counts and highlight uncovered regions.

Describe alternatives you've considered
I looked to see if Coverage Gutters supported reading regions from an existing supported file type, but it appears only to support reading line coverage and branch coverage. (While branch data can help answer some of the same questions, it requires different instrumentation and isn't as easy to visualize.)

[ryanluker]

Thanks for the issue @rkjnsn !

I'm using llvm-cov, which supports generating region-based coverage in addition to function- and line-based coverage. This is useful to see when a part of a line is never executed, such as due to a short-circuiting operator or conditional expression.

Just for my understanding, the region based coverage you are mentioning is this area in the documentation correct?
https://llvm.org/docs/CoverageMappingFormat.html#mapping-region

While branch data can help answer some of the same questions, it requires different instrumentation and isn't as easy to visualize

I was about to mention this, but I see you are one step ahead!

(I will also take a look at the linked pull request, in the next few days)