[clang] Replace SmallStr.str().str() with std::string conversion operator.

Use the std::string conversion operator introduced in
d7049213d0fcda691c9e79f9b41e357198d99738.
diff --git a/clang/tools/clang-refactor/TestSupport.cpp b/clang/tools/clang-refactor/TestSupport.cpp
index 617a671..02b816d 100644
--- a/clang/tools/clang-refactor/TestSupport.cpp
+++ b/clang/tools/clang-refactor/TestSupport.cpp
@@ -190,7 +190,7 @@
               const PartialDiagnosticAt &Diag = Err.getDiagnostic();
               llvm::SmallString<100> DiagText;
               Diag.second.EmitToString(getDiags(), DiagText);
-              ErrorMessage = DiagText.str().str();
+              ErrorMessage = std::string(DiagText);
             });
       }
       if (!CanonicalResult && !CanonicalErrorMessage) {