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_builtin_macros/src/assert.rs

@@ -4,7 +4,7 @@ use crate::edition_panic::use_panic_2021;
 use rustc_ast::ptr::P;
 use rustc_ast::token;
 use rustc_ast::tokenstream::{DelimSpan, TokenStream};
-use rustc_ast::{Expr, ExprKind, MacArgs, MacCall, MacDelimiter, Path, PathSegment, UnOp};
+use rustc_ast::{DelimArgs, Expr, ExprKind, MacCall, MacDelimiter, Path, PathSegment, UnOp};
 use rustc_ast_pretty::pprust;
 use rustc_errors::{Applicability, PResult};
 use rustc_expand::base::{DummyResult, ExtCtxt, MacEager, MacResult};
@@ -54,11 +54,11 @@ pub fn expand_assert<'cx>(
             call_site_span,
             ExprKind::MacCall(P(MacCall {
                 path: panic_path(),
-                args: P(MacArgs::Delimited(
-                    DelimSpan::from_single(call_site_span),
-                    MacDelimiter::Parenthesis,
+                args: P(DelimArgs {
+                    dspan: DelimSpan::from_single(call_site_span),
+                    delim: MacDelimiter::Parenthesis,
                     tokens,
-                )),
+                }),
                 prior_type_ascription: None,
             })),
         );