cxxmodules: add support for clang

---

This relies on patches from @ChuanqiXu from here:

• https://reviews.llvm.org/D137534
• https://reviews.llvm.org/D137527
• https://reviews.llvm.org/D137526
• https://reviews.llvm.org/D137059
• https://reviews.llvm.org/D137058

They are pushed and tagged on my LLVM fork:

• https://github.com/mathstuf/llvm-project/tree/p1689r5-csd

TODO:

• The CI image needs built and pushed. (@brad.king); jobs have been canceled but can be rerun when the image is ready.