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

[Shilei Tian via cfe-commits]

shiltian wrote:

* **#181193** <a href="https://app.graphite.com/github/pr/llvm/llvm-project/181193?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a> 👈 <a href="https://app.graphite.com/github/pr/llvm/llvm-project/181193?utm_source=stack-comment-view-in-graphite" target="_blank">(View in Graphite)</a>
* `main`


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