[llvm] [Docs][DebugInfo][RemoveDIs] Document some debug-info transition info (PR #79167)

[J. Ryan Stinnett via llvm-commits]

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+# What's all this then?
+
+We're planning on removing debug-info intrinsics from LLVM, as they're slow, unweildy and can confuse optimisation passes if they're not expecting them. Instead of having a sequence of instructions that looks like this:
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jryans wrote:

```suggestion
We're planning on removing debug-info intrinsics from LLVM, as they're slow, unwieldy and can confuse optimisation passes if they're not expecting them. Instead of having a sequence of instructions that looks like this:
```

https://github.com/llvm/llvm-project/pull/79167