Foundation: repair Darwin compatibility test suite

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

Author: compnerd

Foundation/DateIntervalFormatter.swift

@@ -159,8 +159,8 @@ open class DateIntervalFormatter: Formatter {
         let dateTemplateFromStylesNS = dateTemplateFromStyles?.takeRetainedValue()._nsObject
         aCoder.encode(dateTemplateFromStylesNS, forKey: "NS.dateTemplateFromStyles")
 
-        aCoder.encode(modified, forKey: "NS.modified");
-        aCoder.encode(useTemplate, forKey: "NS.useTemplate")
+        aCoder.encode(modified.boolValue, forKey: "NS.modified");
+        aCoder.encode(useTemplate.boolValue, forKey: "NS.useTemplate")
 
         let localeNS = locale?.takeRetainedValue()._nsObject
         aCoder.encode(localeNS, forKey: "NS.locale")