Issue Description:

The app I'm working on has suddenly started crashing seemingly due to an access race on `_swiftEmptyArrayStorage`. I have never heard of this variable before, but it is mentioned in the crash log:

Location is global '_swiftEmptyArrayStorage' at 0x7fff913c36e0 (libswiftCore.dylib+0x0000337e46f0)

Two utterly unrelated pieces of the code seem to be simultaneously accessing this global variable. One of them does approximately this:

public final class AccessRacer {
    internal var dict: [Int: [String]] = [:]

    public func thisParticipatesInAccessRace (id: Int, string: String) {
        self.dict[id] ?+= [string]
    }
}

infix operator ?+=
public func ?+= <Element> (lhs: inout [Element]?, rhs: [Element]) {
    lhs = lhs ?? []
    lhs? += rhs
}

The top item in that stack trace is `specialized Array.append<A>(contentsOf:)`. This stack trace is the one that is indicated to be doing the reading of the conflicted data. The other stack trace (which is doing the writing to the data) culminates in a function call into a third party library - the top two items in that stack trace are `flip(array:)` and finally `specialized _copySequenceToContiguousArray<A>(_:)`. Based on the information I have this seems like a Swift bug and not a me-bug. If I'm wrong, I'm sorry to waste your time. Thanks very much.