Merge pull request swiftlang#39750 from DougGregor/force-link-objc-back-deployed

Force-link the Objective-C library in the back-deployed concurrency libs

Author: shahmishal

stdlib/public/BackDeployConcurrency/CMakeLists.txt

@@ -46,7 +46,8 @@ add_compile_definitions(SWIFT_CONCURRENCY_BACK_DEPLOYMENT)
 set(swift_concurrency_install_component back-deployment)
 set(swift_concurrency_options
   BACK_DEPLOYMENT_LIBRARY 5.5
-  DARWIN_INSTALL_NAME_DIR "@rpath")
+  DARWIN_INSTALL_NAME_DIR "@rpath"
+  LINK_FLAGS -lobjc)
 set(swift_concurrency_extra_sources
   "../BackDeployConcurrency/Exclusivity.cpp"
   "../BackDeployConcurrency/Metadata.cpp"