[FileManager] _attributesOfItem: use creationDate from the stat extension in NSURL

[finagolfin]

rather than repeating that logic

The last case in stat.creationDate also works for Android.

Note that it uses st_birthtimespec instead on Darwin, let me know if that makes a difference. @compnerd, this pull redoes #2884 because it didn't work on Android and to reuse the existing logic, let me know what you think.

[spevans]

looking at the manpage for stat(2) on macOS, st_birthtimespec does look like the better choice:

 struct timespec st_ctimespec;     /* time of last status change */
 struct timespec st_birthtimespec; /* time of file creation(birth) */

[spevans]

@swift-ci test

[compnerd]

Could you please ensure that this still works on Windows? If it does, LGTM.

[finagolfin]

I don't use Windows so I can't ensure it, but the Windows case being used now exactly matches the else case you added, so it should be fine.

[compnerd]

I was more worried about the property not existing and this failing to build. Seems that the property is present. LGTM.

[compnerd]

Thanks for the cleanup @buttaface!