unsafe fn argument_types(&mut self, func: &SirFunction) -> WeldResult<Vec<LLVMTypeRef>> {
        let mut types = vec![];
        for (_, ty) in func.params.iter() {
            types.push(self.llvm_type(ty)?);
        }
        Ok(types)
    }
Build the list of argument types for an SIR function.
weld/src/codegen/llvm2/mod.rs