Revert r122395, "Fixes file extension for loadable modules on OS X."

In trunk, every users assume add_llvm_loadable_module as "loadable module" and no one sets neither SHARED, ... nor also MODULE!
Unfortunately, all loadable modules were linked as not "MODULE" but "SHARED".

If this change caused any regressions, I wish guys to fix it properly. ;)

llvm-svn: 200762