Revert to using libdispatch to reap threads on MacOSX. Code was accidentally checked in that is now reverted.
Revert to using libdispatch to reap threads on MacOSX. Code was accidentally checked in that is now reverted.

<rdar://problem/25643874>



git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@266118 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb