[SR-5988] Memory leak in libdispatch on Linux

Issue Description:

Running the code below on Linux leaks memory, but it works fine on Darwin. I don't think that the code should be leaking memory (due to the wait). Swapping the `x=x+1` for the `print()` also leaks.

import Dispatch

func foo(q: DispatchQueue, g: DispatchGroup) {
  for _ in 0..<200 {
    q.async(group: g) {
      print()
    }
  }
  g.wait()
}

let g = DispatchGroup()
let q1 = DispatchQueue(label: "com.queue", attributes: .concurrent)

while true {
  foo(q: q1, g:g)
}

swift --version
Swift version 4.0 (swift-4.0-RELEASE)
Target: x86_64-unknown-linux-gnu

My investigations suggest that the following calloc of 64 bytes is never freed:

0 libc-2.23.so 0x00007fe837b88fba __calloc + 682 at malloc.c:3199
1 libdispatch.so 0x00007fe83a195854 _dispatch_continuation_alloc_from_heap + 36
2 libdispatch.so 0x00007fe83a1a2e5f dispatch_group_async + 207
3 libdispatch.so 0x00007fe83a1b7b9a Dispatch.DispatchQueue.async(group: Swift.Optional<Dispatch.DispatchGroup>, qos: Dispatch.DispatchQoS, flags: Dispatch.DispatchWorkItemFlags, execute: @convention(block) () -> ()) -> () + 31

Start by running the provided Swift 4 reproducer on Linux and examining the allocation path through dispatch_group_async and _dispatch_continuation_alloc_from_heap in libdispatch. Compare the behavior with Darwin and confirm that repeated calls to foo no longer cause memory growth or leave the reported 64-byte allocation unreleased.