Generate compile_commands.json for clangd

[HKalbasi]

There is a json compilation database format that c/c++ tools like clangd (lsp for c/c++, like rust-analyzer for rust) can use to understand c/c++ projects without knowing about the build system. Some build systems like cmake have first class support for it. It would be great to support an option for emitting it inside build.rs. There is a tool called bear that can proxy the calls to the compiler and emit that json but updating at each build without running a separate tool is even better.

[ldesgoui]

Dupe of #497