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_parse/src/lib.rs

@@ -62,7 +62,7 @@ pub fn parse_crate_from_file<'a>(input: &Path, sess: &'a ParseSess) -> PResult<'
 pub fn parse_crate_attrs_from_file<'a>(
     input: &Path,
     sess: &'a ParseSess,
-) -> PResult<'a, Vec<ast::Attribute>> {
+) -> PResult<'a, ast::AttrVec> {
     let mut parser = new_parser_from_file(sess, input, None);
     parser.parse_inner_attributes()
 }
@@ -79,7 +79,7 @@ pub fn parse_crate_attrs_from_source_str(
     name: FileName,
     source: String,
     sess: &ParseSess,
-) -> PResult<'_, Vec<ast::Attribute>> {
+) -> PResult<'_, ast::AttrVec> {
     new_parser_from_source_str(sess, name, source).parse_inner_attributes()
 }