Unbreak build, bring in std::string for GCC 4.3

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66876 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/TableGen/TGSourceMgr.h b/utils/TableGen/TGSourceMgr.h
index 33d3c10..257b950 100644
--- a/utils/TableGen/TGSourceMgr.h
+++ b/utils/TableGen/TGSourceMgr.h
@@ -16,6 +16,7 @@
 
 #include <cassert>
 #include <vector>
+#include <string>
 
 namespace llvm {
   class MemoryBuffer;