[stdlib] Do not allocate when creating a Set from a generic Sequence which happens to be another Set

[karwa]

Previous implementation calls .init(minimumCapacity:), allocating storage, and then sets the _variant, which deallocates that storage.

Also, the comment made it sound like this only applied to native Sets (perhaps excluding bridged sets?), but that does not appear to be the case. _variant is a plain, stored property - it will always be overwritten with the downcasted Set's variant.

[karwa]

@lorentey could you take a look?

[lorentey]

Oh good catch 💯

[lorentey]

@swift-ci test

[lorentey]

Also, the comment made it sound like this only applied to native Sets (perhaps excluding bridged sets?), but that does not appear to be the case. _variant is a plain, stored property - it will always be overwritten with the downcasted Set's variant.

I suspect the original intention was not to adopt the storage of bridged NSSets, so that this initializer would always return a native set instance. Then again, I don't think there is a need to implement this -- the current semantics are fine. (The canonical way to convert a bridged set to native is to call reserveCapacity(0) on it, not to call this initializer.)