Add regression test for rust-lang#93911

Signed-off-by: Yuki Okushi <[email protected]>

tests/ui/lifetimes/issue-93911.rs

@@ -0,0 +1,18 @@
+// check-pass
+// edition:2021
+
+#![allow(dead_code)]
+
+struct Foo<'a>(&'a u32);
+
+impl<'a> Foo<'a> {
+    async fn foo() {
+        struct Bar<'b>(&'b u32);
+
+        impl<'b> Bar<'b> {
+            async fn bar() {}
+        }
+    }
+}
+
+fn main() {}