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_parse/src/parser/nonterminal.rs

@@ -36,7 +36,7 @@ impl<'a> Parser<'a> {
         }
 
         match kind {
-            NonterminalKind::Expr2021 => {
+            NonterminalKind::Expr2021 { inferred: _ } => {
                 token.can_begin_expr()
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Let)
@@ -47,7 +47,6 @@ impl<'a> Parser<'a> {
                 token.can_begin_expr()
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Let)
-                && (!token.is_keyword(kw::Const) || token.span.edition().at_least_rust_2024())
             }
             NonterminalKind::Ty => token.can_begin_type(),
             NonterminalKind::Ident => get_macro_ident(token).is_some(),
@@ -149,7 +148,7 @@ impl<'a> Parser<'a> {
                 })?)
             }
 
-            NonterminalKind::Expr | NonterminalKind::Expr2021 => {
+            NonterminalKind::Expr | NonterminalKind::Expr2021 { inferred: _ } => {
                 NtExpr(self.parse_expr_force_collect()?)
             }
             NonterminalKind::Literal => {