Add regression test for rust-lang#75961

Author: vandenheuvel

src/test/ui/trait-bounds/issue-75961.rs

@@ -0,0 +1,7 @@
+// check-pass
+
+pub fn foo<'a>(s: &'a mut ()) where &'a mut (): Clone {
+    <&mut () as Clone>::clone(&s);
+}
+
+fn main() {}