Replace OwningPtr with std::unique_ptr.

This compiles cleanly with lldb/lld/clang-tools-extra/llvm.

llvm-svn: 203279
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index dbc62b7..73b7db7 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -122,7 +122,7 @@
       BuryPointer(TM.release());
   }
 
-  llvm::OwningPtr<TargetMachine> TM;
+  std::unique_ptr<TargetMachine> TM;
 
   void EmitAssembly(BackendAction Action, raw_ostream *OS);
 };