attributes: remove use of non-MSRV-compliant Option::flatten

PR #875 added code to `tracing-attributes` that uses `Option::flatten`,
which was only added to the standard library in Rust 1.40. This broke
our MSRV, but we missed it because the MSRV CI checks weren't working
correctly (fixed in #934).

This commit removes the use of `Option::flatten`, and replaces it with a
manual implementation. It's a little less pretty, but it builds on our
MSRV (1.39.0).

tracing-attributes/src/lib.rs

@@ -929,8 +929,11 @@ fn get_async_trait_info(block: &Block, block_is_async: bool) -> Option<AsyncTrai
 
             None
         })
-        .next()
-        .flatten();
+        .next();
+    let self_type = match self_type {
+        Some(x) => x,
+        None => None,
+    };
 
     Some(AsyncTraitInfo {
         name: fun.sig.ident.to_string(),