Write a compile_commands.json from build_ext

[godlygeek]

Summary of changes

Produce a JSON database of the compiler commands executed while building extension modules as build/compile_commands.json. This is usable by various C and C++ language servers, linters, and IDEs, like clangd, clang-tidy, and CLion. These tools need to understand the header search path and macros passed on the compiler command line in order to correctly interpret source files. In the case of Python extension modules, the developer might not even know all of the compiler flags that are being used, since some are inherited from the interpreter via sysconfig.

Closes #1975

Pull Request Checklist

• Changes have tests
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[godlygeek]

Let's call this a proof-of-concept. There are no tests yet. This implementation changes both the setuptools build_ext command and the vendored distutils.unixccompiler module.

If this seems like a reasonable approach to the problem, I guess the changes to unixccompiler would need to be done in https://github.com/pypa/distutils/ instead, and tested there.

Perhaps it should also apply to non-Unix compilers as well, but I'm not sure. compile_commands.json originated from the Clang/LLVM world, and I think most tools that use it expect clang- or gcc-compatible flags.

I'm not very familiar with the architecture of setuptools, so if this approach seems fundamentally wrong or there's a cleaner way to do it (especially one that doesn't require modifying the vendored distutils), please point me in the right direction.