Step-style support for compile_commands.json generation

[GreyElaina]

Compared to #22012 and #18391, this PR introduces a new b.addCompileCommands along with the corresponding Step implementation. I believe this approach is more flexible than using -fcompdb / -fno-compdb.

The only issue is that I made this thing look ugly, and I can't think of a better way to do it.

I've simply wrapped it into a Step — the original implementation comes from the two PRs I mentioned earlier.

const compile_commands = b.addCompileCommands(b.path("compile_commands.json"));

b.getInstallStep().dependOn(&compile_commands.step);