Include <string> header to allow compiling for MSVC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@51847 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/CGObjCRuntime.h b/lib/CodeGen/CGObjCRuntime.h
index 6ec6e0c..94217dd 100644
--- a/lib/CodeGen/CGObjCRuntime.h
+++ b/lib/CodeGen/CGObjCRuntime.h
@@ -16,6 +16,7 @@
 #ifndef CLANG_CODEGEN_OBCJRUNTIME_H
 #define CLANG_CODEGEN_OBCJRUNTIME_H
 #include "llvm/ADT/SmallVector.h"
+#include <string>
 
 namespace llvm {
   class IRBuilder;