Auto merge of #96546 - nnethercote:overhaul-MacArgs, r=petrochenkov

Overhaul `MacArgs`

Motivation:
- Clarify some code that I found hard to understand.
- Eliminate one use of three places where `TokenKind::Interpolated` values are created.

r? `@petrochenkov`

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,
     }
 }