Add RFC 1238's unsafe_destructor_blind_to_params (UGEH) where needed.

I needed it in `RawVec`, `Vec`, and `TypedArena` for `rustc` to
bootstrap; but of course that alone was not sufficient for `make
check`.

Later I added `unsafe_destructor_blind_to_params` to collections, in
particular `LinkedList` and `RawTable` (the backing representation for
`HashMap` and `HashSet`), to get the regression tests exercising
cyclic structure from PR #27185 building.

----

Note that the feature is `dropck_parametricity` (which is not the same
as the attribute's name). We will almost certainly vary our strategy
here in the future, so it makes some sense to have a not-as-ugly name
for the feature gate. (The attribute name was deliberately selected to
be ugly looking.)

src/test/run-pass/issue-24805-dropck-itemless.rs

@@ -13,6 +13,8 @@
 
 #![allow(non_camel_case_types)]
 
+#![feature(dropck_parametricity)]
+
 trait UserDefined { }
 
 impl UserDefined for i32 { }
@@ -26,7 +28,10 @@ impl<'a, T> UserDefined for &'a T { }
 macro_rules! impl_drop {
     ($Bound:ident, $Id:ident) => {
         struct $Id<T:$Bound>(T);
-        impl <T:$Bound> Drop for $Id<T> { fn drop(&mut self) { } }
+        impl <T:$Bound> Drop for $Id<T> {
+            #[unsafe_destructor_blind_to_params]
+            fn drop(&mut self) { }
+        }
     }
 }