Right now you can't.
You can, however, write an LLVM assembly function separately in its own file, mark it as alwaysinline
, then compile it with the rest of your files - this should get you the same result. See this related question on how to first compile your C files to IR and then link them together.
You can even compile the files separately and make sure you perform link-time optimization (LTO), should do the same thing.