[clang] [llvm] [KeyInstr] Add docs (PR #137991)

[Orlando Cazalet-Hyams via llvm-commits]

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+# Key Instructions debug info in LLVM
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+Key Instructions reduces the jumpiness of optimized code debug stepping. This document explains the feature and how it is implemented in LLVM. For Clang support please see the [Clang docs](../../clang/docs/KeyInstructionsClang.md)
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OCHyams wrote:

(replied to the comment on the other doc - resolving this one to keep it in one place)

https://github.com/llvm/llvm-project/pull/137991