add FIXME to a test which seems wrong.

Author: lcnr

tests/ui/type-alias-impl-trait/coherence_generalization.rs

@@ -1,5 +1,7 @@
 // check-pass
 
+// FIXME(type_alias_impl_trait): What does this test? This needs a comment
+// explaining what we're worried about here.
 #![feature(type_alias_impl_trait)]
 trait Trait {}
 type Opaque<T> = impl Sized;