Save pre-link bitcode to ModuleCodegen

Author: dianqk

src/lib.rs

@@ -393,7 +393,7 @@ impl WriteBackendMethods for GccCodegenBackend {
     unsafe fn optimize(
         _cgcx: &CodegenContext<Self>,
         _dcx: DiagCtxtHandle<'_>,
-        module: &ModuleCodegen<Self::Module>,
+        module: &mut ModuleCodegen<Self::Module>,
         config: &ModuleConfig,
     ) -> Result<(), FatalError> {
         module.module_llvm.context.set_optimization_level(to_gcc_opt_level(config.opt_level));