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

Use the std::string conversion operator introduced in
d7049213d0fcda691c9e79f9b41e357198d99738.
diff --git a/llvm/lib/Support/YAMLParser.cpp b/llvm/lib/Support/YAMLParser.cpp
index db1eb66..4f9feee 100644
--- a/llvm/lib/Support/YAMLParser.cpp
+++ b/llvm/lib/Support/YAMLParser.cpp
@@ -1642,7 +1642,7 @@
   Token T;
   T.Kind = Token::TK_BlockScalar;
   T.Range = StringRef(Start, Current - Start);
-  T.Value = Str.str().str();
+  T.Value = std::string(Str);
   TokenQueue.push_back(T);
   return true;
 }