Add `_dispatch_install_thread_detach_callback()` on all platforms#841

triplef wants to merge 1 commit intoswiftlang:mainswiftlang/swift-corelibs-libdispatch:mainfrom

triplef:expose-thread-detach-callbacktriplef/swift-corelibs-libdispatch:expose-thread-detach-callbackCopy head branch name to clipboard