Note why this test is still run-pass, at least for now

Author: Zalathar

tests/ui/closures/2229_closure_analysis/unresolvable-upvar-issue-87987.rs

@@ -4,6 +4,10 @@
 //! Under the Rust 2021 disjoint capture rules, this sort of non-capture can
 //! occur when a place is only inspected by infallible non-binding patterns.
 
+// FIXME(#135985): On its own, this test should probably just be check-pass.
+// But there are few/no other tests that use non-binding array patterns and
+// invoke the later parts of the compiler, so building/running has some value.
+
 //@ run-pass
 //@ edition:2021