Rollup merge of #126700 - compiler-errors:fragment, r=fmease

Make edition dependent `:expr` macro fragment act like the edition-dependent `:pat` fragment does

Parse the `:expr` fragment as `:expr_2021` in editions <=2021, and as `:expr` in edition 2024. This is similar to how we parse `:pat` as `:pat_param` in edition <=2018 and `:pat_with_or` in >=2021, and means we can get rid of a span dependency from `nonterminal_may_begin_with`.

Specifically, this fixes a theoretical regression since the `expr_2021` macro fragment previously would allow `const {}` if the *caller* is edition 2024. This is inconsistent with the way that the `pat` macro fragment was upgraded, and also leads to surprising behavior when a macro *caller* crate upgrades to edtion 2024, since they may have parsing changes that they never asked for (with no way of opting out of it).

This PR also allows using `expr_2021` in all editions. Why was this was disallowed in the first place? It's purely additive, and also it's still feature gated?

r? ```@fmease``` ```@eholk``` cc ```@vincenzopalazzo```
cc #123865

Tracking:

- https://github.com/rust-lang/rust/issues/123742

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -1292,7 +1292,9 @@ fn is_in_follow(tok: &mbe::TokenTree, kind: NonterminalKind) -> IsInFollow {
                 // maintain
                 IsInFollow::Yes
             }
-            NonterminalKind::Stmt | NonterminalKind::Expr | NonterminalKind::Expr2021 => {
+            NonterminalKind::Stmt
+            | NonterminalKind::Expr
+            | NonterminalKind::Expr2021 { inferred: _ } => {
                 const TOKENS: &[&str] = &["`=>`", "`,`", "`;`"];
                 match tok {
                     TokenTree::Token(token) => match token.kind {

compiler/rustc_expand/src/mbe/quoted.rs

@@ -113,7 +113,8 @@ pub(super) fn parse(
                                                 );
                                                 token::NonterminalKind::Ident
                                             });
-                                    if kind == token::NonterminalKind::Expr2021
+                                    if kind
+                                        == (token::NonterminalKind::Expr2021 { inferred: false })
                                         && !features.expr_fragment_specifier_2024
                                     {
                                         rustc_session::parse::feature_err(