[llvm] [llvm] Avoid 'raw_string_ostream::str()' (NFC) (PR #96995)

[Kazu Hirata via llvm-commits]

================
@@ -148,7 +148,7 @@ std::string CompleteNodeLabelString(
   std::string Str;
   raw_string_ostream OS(Str);
   HandleBasicBlock(OS, *Node);
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kazutakahirata wrote:

Could we rename `Str` to `OutStr` and drop the copy construction?

```suggestion
  std::string OutStr;
  raw_string_ostream OS(OutStr);
  HandleBasicBlock(OS, *Node);
```

https://github.com/llvm/llvm-project/pull/96995