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

[Florian Mayer via llvm-commits]

fmayer 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;
    }
```

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