In Twine::str(), if the Twine stores only a std::string, just return a direct copy...

In Twine::str(), if the Twine stores only a std::string, just return a direct copy of that instead of first copying to a SmallString and converting that to a std::string. Also fix some indentation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135267 91177308-0d34-0410-b5e6-96231b3b80d8