Change CodeGen::Make() to take module and errors as pointers and not references (#5229)

Per [the style
guide](https://github.com/carbon-language/carbon-lang/blob/trunk/docs/project/cpp_style_guide.md#syntax-and-formatting):
* If it is captured and must outlive the call expression itself, use a
pointer and document that it must not be null (unless it is also
optional).
* When storing an object's address as a non-owned member, prefer storing
a pointer.

toolchain/driver/compile_subcommand.cpp

@@ -641,8 +641,9 @@ auto CompilationUnit::PostCompile() -> void {
 }
 
 auto CompilationUnit::RunCodeGenHelper() -> bool {
-  std::optional<CodeGen> codegen = CodeGen::Make(
-      *module_, options_.codegen_options.target, *driver_env_->error_stream);
+  std::optional<CodeGen> codegen =
+      CodeGen::Make(module_.get(), options_.codegen_options.target,
+                    driver_env_->error_stream);
   if (!codegen) {
     return false;
   }