[let_underscore_untyped]: fix false positive on async function

Author: samueltardieu

clippy_lints/src/let_underscore.rs

@@ -200,7 +200,7 @@ impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
                 }
 
                 // Ignore if it is from a procedural macro...
-                if is_from_proc_macro(cx, init) {
+                if is_from_proc_macro(cx, init) || init.span.from_expansion() {
                     return;
                 }
 

tests/ui/let_underscore_untyped.rs

@@ -73,3 +73,5 @@ fn main() {
     #[allow(clippy::let_underscore_untyped)]
     let _ = a();
 }
+
+async fn dont_lint_async_prototype(_: u8) {}