[Concurrency] Fix back deploying concurrency threading (take 2).

Looks like I got the fix slightly wrong.  Sorry.  This should work.

Author: al45tair

stdlib/public/BackDeployConcurrency/CMakeLists.txt

@@ -66,4 +66,4 @@ set(LLVM_OPTIONAL_SOURCES
   TaskSleepDuration.swift)
 
 add_subdirectory(../Concurrency stdlib/public/BackDeployConcurrency)
-add_subdirectory(../Threading stdlib/public/BackDeployConcurrency)
+add_subdirectory(../Threading)