Remove superfluous .str() and replace std::string concatenation with Twine.

llvm-svn: 233392
diff --git a/llvm/lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp b/llvm/lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp
index d2b4eec..8c9217b 100644
--- a/llvm/lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp
@@ -40,7 +40,7 @@
   if (Filename.find(':') == 1)
     Filepath = Filename;
   else
-    Filepath = (Dir + Twine("\\") + Filename).str();
+    Filepath = (Dir + "\\" + Filename).str();
 
   // Canonicalize the path.  We have to do it textually because we may no longer
   // have access the file in the filesystem.