[llvm] [docs] Strengthen our quality standards and connect AI contribution policy to it (PR #154441)

[Reid Kleckner via llvm-commits]

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+# LLVM AI Tool Use Policy
+
+## Policy
+
+LLVM's policy is that contributors can use whatever tools they would like to
+craft their contributions, but there must be a **human in the loop**.
+**Contributors must read and review all LLM-generated code or text before they
+ask other project members to review it.** The contributor is always the author
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rnk wrote:

I suspect this is where we're going to end up, but I want to leave it out of scope of this PR. I got positive feedback in the LLVM project council meeting, and I don't want to edit too much more before landing this basis and then adding the exceptions that I fully expect to need to add.

https://github.com/llvm/llvm-project/pull/154441