[clang] [clang][docs] Generate diagnostics reference as Markdown (PR #221361)

[Shengxin Pei via cfe-commits]

https://github.com/TPPPP72 commented:

I’ve looked through everything except the `.cpp` files (since I’m not familiar with TableGen), and I don’t see any issues. 
Perhaps other reviewers only need to take a quick look at the parts other than the C++ code.

https://github.com/llvm/llvm-project/pull/221361