Always default `optionalsDefaultNil` to `false`, despite Swift 5

[gohanlon]

Description

Swift provides non-public initializers that default optionals to nil. A design tenet of MemberwiseInit has been to strive to be a pure superset of Swift's memberwise initializer, and this approach has largely served it well.

However, I'm convinced that MemberwiseInit should deviate from Swift 5 and always default _optionalsDefaultNil to false, regardless of access level.

Swift's behavior is:

• complex and often not familiar to developers,
• lamented by (at least one of) its designers,
• different for Int? and Optional<Int> (defaulted nil and not defaulted, respectively),
• hasn't been changeable due to Swift's source compatibility principles, but could be “corrected” in Swift 6. [Update: Swift 6 didn't change this behavior.]

MemberwiseInit can offer a more straightforward approach: Optionals should not default to nil unless explicitly specified by the developer.

This has been debated at length on the forums:

• https://forums.swift.org/t/unexpected-memberwise-inits-for-all-optional-structs/41169
• https://forums.swift.org/t/pre-pitch-remove-the-implicit-initialization-of-optional-variables/52300
• https://forums.swift.org/t/pitch-remove-default-initialization-of-optional-bindings/6961