Add string to try and fix build from r167629.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@167631 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Chad Rosier

include/clang/Basic/SourceLocation.h

@@ -21,6 +21,7 @@
 #include <utility>
 #include <functional>
 #include <cassert>
+#include <string>
 
 namespace llvm {
   class MemoryBuffer;