Add test for uniquifying regions

tests/ui/traits/new-solver/iter-filter-projection.rs

@@ -0,0 +1,12 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+use std::{iter, slice};
+
+struct Attr;
+
+fn test<'a, T: Iterator<Item = &'a Attr>>() {}
+
+fn main() {
+    test::<iter::Filter<slice::Iter<'_, Attr>, fn(&&Attr) -> bool>>();
+}