fn fat_lto(
cgcx: &CodegenContext<LlvmCodegenBackend>,
dcx: DiagCtxtHandle<'_>,
modules: Vec<FatLtoInput<LlvmCodegenBackend>>,
serialized_modules: Vec<(SerializedModule<ModuleBuffer>, CString)>,
symbols_below_threshold: &[*const c_char],
) -> ModuleCodegen<ModuleLlvm>