Merge pull request swiftlang#59227 from al45tair/pr/fix-bd-concurrency-2

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

Author: ahoppen

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)