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_expand/src/module.rs

@@ -1,6 +1,6 @@
 use crate::base::ModuleData;
 use rustc_ast::ptr::P;
-use rustc_ast::{token, Attribute, Inline, Item, ModSpans};
+use rustc_ast::{token, AttrVec, Attribute, Inline, Item, ModSpans};
 use rustc_errors::{struct_span_err, DiagnosticBuilder, ErrorGuaranteed};
 use rustc_parse::new_parser_from_file;
 use rustc_parse::validate_attr;
@@ -48,7 +48,7 @@ pub(crate) fn parse_external_mod(
     span: Span, // The span to blame on errors.
     module: &ModuleData,
     mut dir_ownership: DirOwnership,
-    attrs: &mut Vec<Attribute>,
+    attrs: &mut AttrVec,
 ) -> ParsedExternalMod {
     // We bail on the first error, but that error does not cause a fatal error... (1)
     let result: Result<_, ModError<'_>> = try {
@@ -63,9 +63,9 @@ pub(crate) fn parse_external_mod(
 
         // Actually parse the external file as a module.
         let mut parser = new_parser_from_file(&sess.parse_sess, &mp.file_path, Some(span));
-        let (mut inner_attrs, items, inner_span) =
+        let (inner_attrs, items, inner_span) =
             parser.parse_mod(&token::Eof).map_err(|err| ModError::ParserError(err))?;
-        attrs.append(&mut inner_attrs);
+        attrs.extend(inner_attrs);
         (items, inner_span, mp.file_path)
     };
     // (1) ...instead, we return a dummy module.