swift234

[CMake] Revert SwiftARCPasses back to add_llvm_loadable_module.

This is necessary to not link in a duplicate copy of LLVMSupport. This is
the same interface as LLVM's example modules and Clang's sample plug-ins.

Fariborz, if this breaks your Xcode setup again, please let me know and I'll
come take a look. (The Clang targets work fine for me in Xcode.)

Swift SVN r14547