[llvm] [msan] Add debugging for handleUnknownIntrinsic (PR #123381)

[Thurston Dang via llvm-commits]

thurstond wrote:

> would it be easier to just make it this in the caller:
> 
> ```
>     default:
>       if (handleUnknownIntrinsic(I)) {
>            if (ClDumpStrictIntrinsics)
>              dumpInst(I);
>          LLVM_DEBUG(dbgs() << "UNKNOWN INTRINSIC HANDLED HEURISTICALLY: " << I
>       } else {
>         visitInstruction(I);
>       }
>       break;
>     }
> ```

I want to avoid adding too much inline code to the `visitIntrinsicInst` function, and I also want `handleUnknownIntrinsic` to include the logging (in case anyone ever calls it from elsewhere); it follows the pattern of `visitInstruction`.

How about the revised approach in https://github.com/llvm/llvm-project/pull/123381/commits/3cba7d7cd6f4847ea5813338e1f4e47e44557a8e, which adds a helper function?

https://github.com/llvm/llvm-project/pull/123381