Remove `NtExpr` and `NtLiteral`.

Notes about tests:
- tests/ui/rfcs/rfc-2294-if-let-guard/feature-gate.rs: some messages are
  now duplicated due to repeated parsing.

- tests/ui/rfcs/rfc-2497-if-let-chains/disallowed-positions.rs: ditto.

- `tests/ui/proc-macro/macro-rules-derive-cfg.rs`: the diff looks large
  but the only difference is the insertion of a single
  invisible-delimited group around a metavar.

- `tests/ui/attributes/nonterminal-expansion.rs`: a slight span
  degradation, somehow related to the recent massive attr parsing
  rewrite (#135726). I couldn't work out exactly what is going wrong,
  but I don't think it's worth holding things up for a single slightly
  suboptimal error message.

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/ui/conditional-compilation/cfg-attr-syntax-validation.rs b/tests/ui/conditional-compilation/cfg-attr-syntax-validation.rs
index 416145a0c15..47418b4e091 100644
--- a/tests/ui/conditional-compilation/cfg-attr-syntax-validation.rs
+++ b/tests/ui/conditional-compilation/cfg-attr-syntax-validation.rs
@@ -28,7 +28,7 @@ struct S9;
 macro_rules! generate_s10 {
     ($expr: expr) => {
         #[cfg(feature = $expr)]
-        //~^ ERROR expected unsuffixed literal, found expression `concat!("nonexistent")`
+        //~^ ERROR expected unsuffixed literal, found `expr` metavariable
         struct S10;
     }
 }