coverage: Make unexpansion of closure bodies more precise

This improves the coverage instrumentation of closures declared in macros, as
seen in `closure_macro.rs` and `closure_macro_async.rs`.

tests/coverage/closure_macro_async.rs

@@ -16,7 +16,7 @@ macro_rules! bail {
 macro_rules! on_error {
     ($value:expr, $error_message:expr) => {
         $value.or_else(|e| {
-            // FIXME(85000): no coverage in closure macros
+            // This closure, which is declared in a macro, should be instrumented.
             let message = format!($error_message, e);
             if message.len() > 0 {
                 println!("{}", message);