Issue Description:

On Linux, calling isValidDate on a DateComponents instance unexpectedly returns false if the timeZone property of the DateComponents instance is different than the timeZone property of its calendar.

Similarly, calling isValidDate(in: ) on a DateComponents instance unexpectedly returns false if the timeZone property of the DateComponents instance is different than the timeZone property of the specified calendar.

This is unexpected because:

• it is inconsistent with Swift 5.0 on Linux

• it is inconsistent with Swift on macOS (both Swift 5.0 and 5.1)

• the API doc states: "If the time zone property is set in the DateComponents, it is used."

Steps to reproduce

import Foundation

let calendarTimeZone = TimeZone(secondsFromGMT: 0)!
let dateComponentsTimeZone = TimeZone(secondsFromGMT: 3600)!

var calendar = Calendar(identifier: .gregorian)
calendar.timeZone = calendarTimeZone

var dc = DateComponents()
dc.calendar = calendar
dc.timeZone = dateComponentsTimeZone
dc.year = 2019
dc.month = 1
dc.day = 2
dc.hour = 3
dc.minute = 4
dc.second = 5
dc.nanosecond = 6

// Should be true.
print("dc.isValidDate = \(dc.isValidDate)")

// Should be true.
print("dc.isValidDate(in: calendar) = \(dc.isValidDate(in: calendar))")

Expected output (and output on macOS)

dc.isValidDate = true
dc.isValidDate(in: calendar) = true

Actual output (Linux)

dc.isValidDate = false
dc.isValidDate(in: calendar) = false

Environment

Linux (exhibits the problem):

• Ubuntu 18.04 server

• Swift version 5.1 (swift-5.1-RELEASE) Target: x86_64-unknown-linux-gnu

macOS (does not exhibit the problem):

• macOS 10.14.6

• Apple Swift version 5.1 (swiftlang-1100.0.270.13 clang-1100.0.33.7)
  Target: x86_64-apple-darwin18.7.0