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

[Shilei Tian via cfe-commits]

shiltian wrote:

Now the documentation has been cross referenced with AMD's official GPUOpen Machine-Readable ISA (https://gpuopen.com/machine-readable-isa/).

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