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

[Orlando Cazalet-Hyams via llvm-commits]

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+# Key Instructions in Clang
+
+Key Instructions is an LLVM feature that reduces the jumpiness of optimized code debug stepping. This document explains how Clang applies the necessary metadata.
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OCHyams wrote:

Couldn't find better wording than yours, how does this look? I fear it's become a little bit of a word salad.

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