Build the back-deployed _Concurrency library with an @rpath install name

When linking against the back-deployed _Concurrency library, ensure that
clients record an `@rpath`-relative dependency.

Author: DougGregor

stdlib/public/BackDeployConcurrency/CMakeLists.txt

@@ -39,6 +39,8 @@ endforeach()
 # Build the concurrency library for back deployment.
 add_compile_definitions(SWIFT_CONCURRENCY_BACK_DEPLOYMENT)
 set(swift_concurrency_install_component back-deployment)
-set(swift_concurrency_options BACK_DEPLOYMENT_LIBRARY 5.5)
+set(swift_concurrency_options
+  BACK_DEPLOYMENT_LIBRARY 5.5
+  DARWIN_INSTALL_NAME_DIR "@rpath")
 
 add_subdirectory(../Concurrency stdlib/public/BackDeployConcurrency)