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

Use the std::string conversion operator introduced in
d7049213d0fcda691c9e79f9b41e357198d99738.
diff --git a/llvm/unittests/Support/FileUtilitiesTest.cpp b/llvm/unittests/Support/FileUtilitiesTest.cpp
index 7fbb601..cf1453b 100644
--- a/llvm/unittests/Support/FileUtilitiesTest.cpp
+++ b/llvm/unittests/Support/FileUtilitiesTest.cpp
@@ -38,7 +38,8 @@
 
   SmallString<128> FinalTestfilePath(RootTestDirectory);
   sys::path::append(FinalTestfilePath, "foo.txt");
-  const std::string TempUniqTestFileModel = FinalTestfilePath.str().str() + "-%%%%%%%%";
+  const std::string TempUniqTestFileModel =
+      std::string(FinalTestfilePath) + "-%%%%%%%%";
   const std::string TestfileContent = "fooFOOfoo";
 
   llvm::Error Err = llvm::writeFileAtomically(TempUniqTestFileModel, FinalTestfilePath, TestfileContent);