Rollup merge of #94368 - c410-f3r:metaaaaaaaaaaaaaaaaaaaaaaaaaaa, r=petrochenkov

[1/2] Implement macro meta-variable expressions

See https://github.com/rust-lang/rust/pull/93545#issuecomment-1050963295

The logic behind `length`, `index` and `count` was removed but the parsing code is still present, i.e., everything is simply ignored like `ignored`.

r? ``@petrochenkov``

compiler/rustc_span/src/symbol.rs

@@ -846,6 +846,7 @@ symbols! {
         macro_export,
         macro_lifetime_matcher,
         macro_literal_matcher,
+        macro_metavar_expr,
         macro_reexport,
         macro_use,
         macro_vis_matcher,