This function is very similar to add_llvm_library(), so this patch turns
add_llvm_loadable_module() into a simple wrapper around add_llvm_library().
In addition to simplifying the code, this makes it possible to mark loadable modules
as BULIDTREE_ONLY. Before this patch modules would be installed unconditionally
which is not always what we want. See https://reviews.llvm.org/D50668.