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/context.rs

@@ -3,7 +3,7 @@ use rustc_ast::{
     ptr::P,
     token,
     tokenstream::{DelimSpan, TokenStream, TokenTree},
-    BinOpKind, BorrowKind, Expr, ExprKind, ItemKind, MacArgs, MacCall, MacDelimiter, MethodCall,
+    BinOpKind, BorrowKind, DelimArgs, Expr, ExprKind, ItemKind, MacCall, MacDelimiter, MethodCall,
     Mutability, Path, PathSegment, Stmt, StructRest, UnOp, UseTree, UseTreeKind, DUMMY_NODE_ID,
 };
 use rustc_ast_pretty::pprust;
@@ -181,11 +181,11 @@ impl<'cx, 'a> Context<'cx, 'a> {
             self.span,
             ExprKind::MacCall(P(MacCall {
                 path: panic_path,
-                args: P(MacArgs::Delimited(
-                    DelimSpan::from_single(self.span),
-                    MacDelimiter::Parenthesis,
-                    initial.into_iter().chain(captures).collect::<TokenStream>(),
-                )),
+                args: P(DelimArgs {
+                    dspan: DelimSpan::from_single(self.span),
+                    delim: MacDelimiter::Parenthesis,
+                    tokens: initial.into_iter().chain(captures).collect::<TokenStream>(),
+                }),
                 prior_type_ascription: None,
             })),
         )