[Diagnostics] NFC: Properly declare CSDiag.h dependencies

Author: xedin

lib/Sema/CSDiag.h

@@ -18,8 +18,16 @@
 #ifndef SWIFT_SEMA_CSDIAG_H
 #define SWIFT_SEMA_CSDIAG_H
 
+#include "ConstraintSystem.h"
+#include "swift/AST/DiagnosticEngine.h"
+#include "llvm/ADT/StringRef.h"
+#include <string>
+
 namespace swift {
-  
+  class Expr;
+  class Type;
+  class SourceLoc;
+
   std::string getTypeListString(Type type);
 
   /// Rewrite any type variables & archetypes in the specified type with