[NFC] Forward-declare DiagGroupID instead of including

Author: AnthonyLatsis

include/swift/AST/DiagnosticEngine.h

@@ -21,7 +21,6 @@
 #include "swift/AST/ActorIsolation.h"
 #include "swift/AST/DeclNameLoc.h"
 #include "swift/AST/DiagnosticConsumer.h"
-#include "swift/AST/DiagnosticGroups.h"
 #include "swift/AST/TypeLoc.h"
 #include "swift/Basic/PrintDiagnosticNamesMode.h"
 #include "swift/Basic/Statistic.h"
@@ -68,6 +67,8 @@ namespace swift {
   /// this enumeration type that uniquely identifies it.
   enum class DiagID : uint32_t;
 
+  enum class DiagGroupID : uint16_t;
+
   /// Describes a diagnostic along with its argument types.
   ///
   /// The diagnostics header introduces instances of this type for each