[clang] [Clang][TableGen] Add documentation generation infrastructure for builtins (PR #181573)

[Shilei Tian via cfe-commits]

shiltian wrote:

Oh I see. I did check the code in `clang/utils/TableGen/ClangAttrEmitter.cpp`. There is not too much value of doing that since the shared code is really not that much.

https://github.com/llvm/llvm-project/pull/181573