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

[Jeremy Morse 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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jmorse wrote:

Again, a sentence indicating what we're positively adding (more information about the significance of a source-location) would help.

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