Provide more specifics about headers in compile_commands.json

[keith]

While debugging the perf of this tool in our project I discovered this:

https://github.com/hedronvision/bazel-compile-commands-extractor/blob/1e08f8e0507b6b6b1f4416a9a22cf5c28beaba93/refresh.template.py#L665-L667

It sounds like there might be some poor devx cases being solved here, but it's not entirely clear to me if it's something we will be affected by or not. Do you remember some more specifics I could test on our project to determine if this time is worth spending? For reference >75% of the generating time is eliminated if we don't generate commands for headers, which is pretty tempting for us. Also we're migrating from cmake and the cmake generated file doesn't include headers either, so I'm definitely interested in the bazel specifics that cause this.