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

[Jeremy Morse via llvm-commits]

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+# Key Instructions debug info in LLVM
+
+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)
+
+## Status
+
+In development, but mostly complete. The feature is currently disabled for coroutines.
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jmorse wrote:

Can we upgrade to "Complete for everything but coroutines"?

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