Create compile_commands.json for integration with other build systems and IDEs

[Doron Behar]

Many build systems, IDEs and text editors plugins, integrate with source
code files using a compilation database. It is `compile_commands.json`
and it is placed in a project's root directory.

It is best explained here:
https://clang.llvm.org/docs/JSONCompilationDatabase.html

As listed here:
(https://github.com/cquery-project/cquery/wiki/compile_commands.json),
some build systems support it out of the box (ninja, cmake and waf) -
(https://ninja-build.org/ https://cmake.org https://waf.io/)

With probably the most common one, the GNU build system, this is not
supported out of the box. Although there is currently 1 workaround
(https://github.com/rizsotto/scan-build), It would be great it would be
supported right out of the box, perhaps by introducing a new flag like
`--compdb` which will create this file.

I think it would be worth the effort of the developers, what do you guys
think?