Foundation: repair Darwin compatibility test suite

Explicitly convert the field to `Bool` from `DarwinBoolean` for encoding
to ensure serialisability.

Author: compnerd

Foundation/DateIntervalFormatter.swift

@@ -158,10 +158,15 @@ open class DateIntervalFormatter: Formatter {
 
         let dateTemplateFromStylesNS = dateTemplateFromStyles?.takeRetainedValue()._nsObject
         aCoder.encode(dateTemplateFromStylesNS, forKey: "NS.dateTemplateFromStyles")
-        
+
+#if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
+        aCoder.encode(modified.boolValue, forKey: "NS.modified");
+        aCoder.encode(useTemplate.boolValue, forKey: "NS.useTemplate")
+#else
         aCoder.encode(modified, forKey: "NS.modified");
         aCoder.encode(useTemplate, forKey: "NS.useTemplate")
-        
+#endif
+
         let localeNS = locale?.takeRetainedValue()._nsObject
         aCoder.encode(localeNS, forKey: "NS.locale")