Use AttrVec in more places.

In some places we use `Vec<Attribute>` and some places we use
`ThinVec<Attribute>` (a.k.a. `AttrVec`). This results in various points
where we have to convert between `Vec` and `ThinVec`.

This commit changes the places that use `Vec<Attribute>` to use
`AttrVec`. A lot of this is mechanical and boring, but there are
some interesting parts:
- It adds a few new methods to `ThinVec`.
- It implements `MapInPlace` for `ThinVec`, and introduces a macro to
  avoid the repetition of this trait for `Vec`, `SmallVec`, and
  `ThinVec`.

Overall, it makes the code a little nicer, and has little effect on
performance. But it is a precursor to removing
`rustc_data_structures::thin_vec::ThinVec` and replacing it with
`thin_vec::ThinVec`, which is implemented more efficiently.

compiler/rustc_builtin_macros/src/test.rs

@@ -227,7 +227,8 @@ pub fn expand_test_or_bench(
             )),
             // #[rustc_test_marker]
             cx.attribute(cx.meta_word(attr_sp, sym::rustc_test_marker)),
-        ],
+        ]
+        .into(),
         // const $ident: test::TestDescAndFn =
         ast::ItemKind::Const(
             ast::Defaultness::Final,
@@ -334,7 +335,7 @@ pub fn expand_test_or_bench(
     });
 
     // extern crate test
-    let test_extern = cx.item(sp, test_id, vec![], ast::ItemKind::ExternCrate(None));
+    let test_extern = cx.item(sp, test_id, ast::AttrVec::new(), ast::ItemKind::ExternCrate(None));
 
     tracing::debug!("synthetic test item:\n{}\n", pprust::item_to_string(&test_const));