Allow this to compile on compilers where <string>

doesn't imply <cassert>.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@39778 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Basic/Diagnostic.h

@@ -15,6 +15,7 @@
 #define LLVM_CLANG_DIAGNOSTIC_H
 
 #include <string>
+#include <cassert>
 
 namespace clang {
   class DiagnosticClient;