Preserve the function table explicitly (#1970)

The main gc pass of unused items in wasm-bindgen was accidentally
removing the function table because we weren't properly rooting it in
the auxiliary section which has a few ways that imports can reference
the function table via intrinsics and closures.

Closes #1967

crates/cli/tests/wasm-bindgen/main.rs

@@ -334,3 +334,21 @@ $",
     )?);
     Ok(())
 }
+
+#[test]
+fn function_table_preserved() {
+    let (mut cmd, _out_dir) = Project::new("function_table_preserved")
+        .file(
+            "src/lib.rs",
+            r#"
+                use wasm_bindgen::prelude::*;
+
+                #[wasm_bindgen]
+                pub fn bar() {
+                    Closure::wrap(Box::new(|| {}) as Box<dyn Fn()>);
+                }
+            "#,
+        )
+        .wasm_bindgen("");
+    cmd.assert().success();
+}