Fix C# glue generation for enums with negative values

[RedworkDE]

Fixes #73004

The error can also happen with 4.x, not just on 3.x.

Test patch I used to trigger this error:

diff --git a/servers/text_server.h b/servers/text_server.h
index af6efb8c7dd..0cb1103722d 100644
--- a/servers/text_server.h
+++ b/servers/text_server.h
@@ -70,7 +70,7 @@ public:
 	};
 
 	enum Orientation {
-		ORIENTATION_HORIZONTAL,
+		ORIENTATION_HORIZONTAL = -1,
 		ORIENTATION_VERTICAL
 	};

Cherry-picks fairly cleanly onto 3.x (it's just a small fix)

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.0.4.

[akien-mga]

There are some conflicts when cherry-picking, I prefer not to hazard resolving them without fully understanding the code, so a dedicated PR would be welcome for 3.x if this is an important bugfix.