[SR-10904] DarwinCompatibility: Unit.init(symbol:) should not be a required initialiser

[spevans]

Previous ID	SR-10904
Radar	None
Original Reporter	@spevans
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug, Linux
Assignee	None
Priority	Medium

md5: 0df819b3842f101923c78bdfa4d29780

Issue Description:

According to https://developer.apple.com/documentation/foundation/unit/1690760-init

Unit.init(symbol:)

should not have a required modifier as it does here: https://github.com/apple/swift-corelibs-foundation/blob/e1071a9d90686b74faeabb9b6611cbdc1945d388/Foundation/Unit.swift#L155

However removing the modifier leads to other issues:

This line https://github.com/apple/swift-corelibs-foundation/blob/e1071a9d90686b74faeabb9b6611cbdc1945d388/Foundation/Measurement.swift#L324

[spevans]

cc @millenomi

[millenomi]

This is an acceptable break from Darwin API as it allows the whole to work; Darwin is fundamentally violating constructor assumptions here.