Auto merge of #118958 - c410-f3r:concat-again, r=petrochenkov

Add a new concat metavar expr

Revival of #111930

Giving it another try now that #117050 was merged.

With the new rules, meta-variable expressions must be referenced with a dollar sign (`$`) and this can cause misunderstands with `$concat`.

```rust
macro_rules! foo {
    ( $bar:ident ) => {
        const ${concat(VAR, bar)}: i32 = 1;
    };
}

// Will produce `VARbar` instead of `VAR_123`
foo!(_123);
```

In other words, forgetting the dollar symbol can produce undesired outputs.

cc #29599
cc https://github.com/rust-lang/rust/issues/124225

compiler/rustc_feature/src/unstable.rs

@@ -516,6 +516,8 @@ declare_features! (
     (unstable, lint_reasons, "1.31.0", Some(54503)),
     /// Give access to additional metadata about declarative macro meta-variables.
     (unstable, macro_metavar_expr, "1.61.0", Some(83527)),
+    /// Provides a way to concatenate identifiers using metavariable expressions.
+    (unstable, macro_metavar_expr_concat, "CURRENT_RUSTC_VERSION", Some(124225)),
     /// Allows `#[marker]` on certain traits allowing overlapping implementations.
     (unstable, marker_trait_attr, "1.30.0", Some(29864)),
     /// Allows exhaustive pattern matching on types that contain uninhabited types in cases that are