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

[Pranav Kant 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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pranavk wrote:

The QEMU AI policy, though much stricter, has section on [Exceptions](https://www.qemu.org/docs/master/devel/code-provenance.html#exceptions). Perhaps it may be a good idea to have something similar to be explicit that LLVM welcomes discussion on exceptions via the RFC process.

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