Make self mutating in init()

[DevAndArtist (Adrian Meister)]

This isn‘t a question about mutability inside init, this is the factory pattern which isn‘t yet supported for classes through the init.

Allow `self = x` in class convenience initializers

In https://github.com/apple/swift/pull/19151, I'm changing code generation for class convenience initializers so that they only have "allocating" entry points, which are responsible for allocating and initializing the entire object. (See Changing class convenience initializers to perform whole object allocation and @objc interop for more background.) With this change, convenience initializers are nearly identical to value or protocol extension initializers at the implementation level, since the s…