Overhaul MacArgs::Eq.

The value in `MacArgs::Eq` is currently represented as a `Token`.
Because of `TokenKind::Interpolated`, `Token` can be either a token or
an arbitrary AST fragment. In practice, a `MacArgs::Eq` starts out as a
literal or macro call AST fragment, and then is later lowered to a
literal token. But this is very non-obvious. `Token` is a much more
general type than what is needed.

This commit restricts things, by introducing a new type `MacArgsEqKind`
that is either an AST expression (pre-lowering) or an AST literal
(post-lowering). The downside is that the code is a bit more verbose in
a few places. The benefit is that makes it much clearer what the
possibilities are (though also shorter in some other places). Also, it
removes one use of `TokenKind::Interpolated`, taking us a step closer to
removing that variant, which will let us make `Token` impl `Copy` and
remove many "handle Interpolated" code paths in the parser.

Things to note:
- Error messages have improved. Messages like this:
  ```
  unexpected token: `"bug" + "found"`
  ```
  now say "unexpected expression", which makes more sense. Although
  arbitrary expressions can exist within tokens thanks to
  `TokenKind::Interpolated`, that's not obvious to anyone who doesn't
  know compiler internals.
- In `parse_mac_args_common`, we no longer need to collect tokens for
  the value expression.

Author: nnethercote

clippy_utils/src/ast_utils.rs

@@ -688,7 +688,8 @@ pub fn eq_mac_args(l: &MacArgs, r: &MacArgs) -> bool {
     match (l, r) {
         (Empty, Empty) => true,
         (Delimited(_, ld, lts), Delimited(_, rd, rts)) => ld == rd && lts.eq_unspanned(rts),
-        (Eq(_, lt), Eq(_, rt)) => lt.kind == rt.kind,
+        (Eq(_, MacArgsEq::Ast(le)), Eq(_, MacArgsEq::Ast(re))) => eq_expr(le, re),
+        (Eq(_, MacArgsEq::Hir(ll)), Eq(_, MacArgsEq::Hir(rl))) => ll.kind == rl.kind,
         _ => false,
     }
 }