Split MacArgs in two.

`MacArgs` is an enum with three variants: `Empty`, `Delimited`, and `Eq`. It's
used in two ways:
- For representing attribute macro arguments (e.g. in `AttrItem`), where all
  three variants are used.
- For representing function-like macros (e.g. in `MacCall` and `MacroDef`),
  where only the `Delimited` variant is used.

In other words, `MacArgs` is used in two quite different places due to them
having partial overlap. I find this makes the code hard to read. It also leads
to various unreachable code paths, and allows invalid values (such as
accidentally using `MacArgs::Empty` in a `MacCall`).

This commit splits `MacArgs` in two:
- `DelimArgs` is a new struct just for the "delimited arguments" case. It is
  now used in `MacCall` and `MacroDef`.
- `AttrArgs` is a renaming of the old `MacArgs` enum for the attribute macro
  case. Its `Delimited` variant now contains a `DelimArgs`.

Various other related things are renamed as well.

These changes make the code clearer, avoids several unreachable paths, and
disallows the invalid values.

compiler/rustc_parse/src/parser/expr.rs

@@ -1501,7 +1501,7 @@ impl<'a> Parser<'a> {
             let lo = path.span;
             let mac = P(MacCall {
                 path,
-                args: self.parse_mac_args()?,
+                args: self.parse_delim_args()?,
                 prior_type_ascription: self.last_type_ascription,
             });
             (lo.to(self.prev_token.span), ExprKind::MacCall(mac))