Enforce T: Hash for Interned<...>

This adds panicking Hash impls for several resolver types that don't
actually satisfy this condition. It's not obvious to me that
rustc_resolve actually upholds the Interned guarantees but fixing that
seems pretty hard (the structures have at minimum some interior
mutability, so it's not really recursively hashable in place...).

compiler/rustc_data_structures/src/intern.rs

@@ -92,7 +92,10 @@ impl<'a, T: Ord> Ord for Interned<'a, T> {
     }
 }
 
-impl<'a, T> Hash for Interned<'a, T> {
+impl<'a, T> Hash for Interned<'a, T>
+where
+    T: Hash,
+{
     #[inline]
     fn hash<H: Hasher>(&self, s: &mut H) {
         // Pointer hashing is sufficient, due to the uniqueness constraint.