SR-14933: Implement copy() in ISO8601DateFormatter.

[YOCKOW]

As with #3008, ISO8601DateFormatter should also implement its own override of copy().

Resolves SR-14933.

[spevans]

@swift-ci test

[spevans]

So interestingly I ran the test_copy on BigSur 11.4.4 using DarwinCompatibilityTests and it fails, which leads me to believe this is actually a bug in Darwin as well.

$ swift
Welcome to Apple Swift version 5.5 (swiftlang-1300.0.20.104 clang-1300.0.21.1).
Type :help for assistance.
  1> import Foundation 
  2.  
  3. let f1 = ISO8601DateFormatter() 
  4. let f2 = f1.copy() as! ISO8601DateFormatter 
  5. print(f1) 
  6. print(f2) 
  7. print(f1 === f2)
<NSISO8601DateFormatter: 0x10040dbd0>
<NSISO8601DateFormatter: 0x10040dbd0>
true

@millenomi Is ISO8601DateFormatter implementing NSCopying correctly on Darwin?

@YOCKOW I think this PR is correct I just want to see Lily's response first. I thinks its also worth adding a XCTAssertFalse(copied === original) into your tests for these copy() fixing PRs.

[YOCKOW]

@spevans
I'm sorry, but I didn't know ISO8601DateFormatter on Darwin has such behavior.
I'm going to modify the test.

[spevans]

@YOCKOW I don't think your PR is wrong, it might be that there is a bug in Darwin's Foundation. I just wanted confirmation that the bug exists on macOS as well. So I would hold off on making any changes for now.

[YOCKOW]

@spevans
Noted with thanks. I've just modified the test in this PR on local, but I won't push it as of now.

[YOCKOW]

Date.ISO8601FormatStyle is coming, but I believe this PR is not stale yet.

[spevans]

@millenomi Could you check my earlier comment in #3009 (comment) it looks like Darwin's Foundation doesn't properly implement ISO8601DateFormatter.copy. I just want to confirm that Darwin has a bug

[millenomi]

It is indeed a bug. Do you happen to have a FB number handy for it?

[spevans]

The sync system seemed to link https://bugs.swift.org/browse/SR-14108 to rdar://problem/73742491, not sure if that is the best bug to cover it, otherwise could link to https://bugs.swift.org/browse/SR-14933 which covers this fix.

[YOCKOW]

Should I file a separate report about that using an other bug tracker such as Feedback Assistant?

[spevans]

@YOCKOW When you get a chance could you possibly backport this fix to the 5.5 branch (and also 5.4 if it can be done before Friday), thanks.