Issue Description:

Consider the following code, which runs error-free in Swift 3.1, but results in a runtime crash in Swift 3.2 and Swift 4:

import Foundation

struct S {
  var str = UUID().uuidString
}

func go() {
    var s = S()
    s = S()

    _ = DispatchQueue.global(qos: .default).async {
        print(s.str)
    }
}

go()
sleep(1)

This crashes using swift that ships with Xcode 9 beta 1 by running it in the following ways:
swift -swift-version 3 file.swift
swift file.swift

Output from the program:

0CC3D5F2-3F86-4B6E-88B6-181891DEF66C
0 swift 0x00000001110823aa PrintStackTraceSignalHandler(void*) + 42
1 swift 0x00000001110817e6 SignalHandler(int) + 662
2 libsystem_platform.dylib 0x00007fffb6d64b3a _sigtramp + 26
3 libsystem_platform.dylib 000000000000000000 _sigtramp + 1227470048
4 libsystem_platform.dylib 0x0000000113ad1744 _sigtramp + 1557580836
5 libswiftCore.dylib 0x00000001167a6770 _swift_release_dealloc + 16
6 libswiftCore.dylib 0x0000000113ad1894 _swift_release_dealloc + 4247957812
7 libsystem_blocks.dylib 0x00007fffb6b89952 _Block_release + 102
8 libdispatch.dylib 0x00007fffb6b1f8fc _dispatch_client_callout + 8
9 libdispatch.dylib 0x00007fffb6b2e196 _dispatch_queue_override_invoke + 743
10 libdispatch.dylib 0x00007fffb6b216b5 _dispatch_root_queue_drain + 476
11 libdispatch.dylib 0x00007fffb6b2148c _dispatch_worker_thread3 + 99
12 libsystem_pthread.dylib 0x00007fffb6d6e5a2 _pthread_wqthread + 1299
13 libsystem_pthread.dylib 0x00007fffb6d6e07d start_wqthread + 13
Killed: 9

In Instruments, you can see that the string held by s is released an extra time.