[clang] [RFC][Docs][Clang][AMDGPU] Add AMDGPU builtins documentation (PR #181193)

[Shilei Tian via cfe-commits]

shiltian wrote:

https://github.com/llvm/llvm-project/pull/181574 is to generate it by TableGen, but we still need to fill in the records.

https://github.com/llvm/llvm-project/pull/181193