Concurrency from the Core project is importing the Darwin platform
overlay, which in turn depends on SwiftCore from the Core project,
breaking the project layering.
Concurrency only needs the Clang module, but Swift does not have a
mechanism to only import a clang module. For now import the
functionality needed from Darwin by importing and wrapping the
associated functions from `<dlfcn.h>` within `CFExecutor.cpp`
Also remove Darwin import from `AsyncStreamBuffer.swift` because it is
not used
