Make Date, DateComponents and DateInterval Sendable

# Motivation
In swiftlang#4777, I made them `Sendable` in the overlay instead of the real `Foundation` module. This PR fixes that.

Author: FranzBusch

Diff for: Sources/Foundation/Date.swift

@@ -14,7 +14,7 @@
 
  A `Date` is independent of a particular calendar or time zone. To represent a `Date` to a user, you must interpret it in the context of a `Calendar`.
  */
-public struct Date : ReferenceConvertible, Comparable, Equatable {
+public struct Date : ReferenceConvertible, Comparable, Equatable, Sendable {
     public typealias ReferenceType = NSDate
 
     fileprivate var _time: TimeInterval

Diff for: Sources/Foundation/DateComponents.swift

@@ -19,7 +19,7 @@
 
  When a new instance of `DateComponents` is created, the date components are set to `nil`.
  */
-public struct DateComponents : ReferenceConvertible, Hashable, Equatable, _MutableBoxing {
+public struct DateComponents : ReferenceConvertible, Hashable, Equatable, Sendable, _MutableBoxing {
     public typealias ReferenceType = NSDateComponents
 
     internal var _handle: _MutableHandle<NSDateComponents>

Diff for: Sources/Foundation/DateInterval.swift

@@ -13,7 +13,7 @@
 @_implementationOnly import CoreFoundation
 
 /// DateInterval represents a closed date interval in the form of [startDate, endDate].  It is possible for the start and end dates to be the same with a duration of 0.  DateInterval does not support reverse intervals i.e. intervals where the duration is less than 0 and the end date occurs earlier in time than the start date.
-public struct DateInterval : ReferenceConvertible, Comparable, Hashable {
+public struct DateInterval : ReferenceConvertible, Comparable, Sendable, Hashable {
     public typealias ReferenceType = NSDateInterval
 
     /// The start date.