Change type of EnumLocaleData to struct (#19193)

Saves allocation and makes the code smaller. It had to be class before we had ref locals.

src/System.Private.CoreLib/shared/System/Globalization/CultureData.Windows.cs

@@ -517,7 +517,7 @@ private static int ConvertFirstDayOfWeekMonToSun(int iTemp)
 
 
         // Context for EnumCalendarInfoExEx callback.
-        private class EnumLocaleData
+        private struct EnumLocaleData
         {
             public string regionName;
             public string cultureName;