Inline const patterns are no longer pre-expansion gated

[fmease]

Found while running fmease/rasur against the UI test suite (that's part of why I've written this parser in the first place, nice!).

---

Example reproducers:

These two programs now get accepted on beta and nightly even though they should continue to be rejected.

macro_rules! analyze { ($p:pat) => {}; }
analyze!(const { 0 });

#[cfg(false)]
fn scope() { let const { 0 }; }

Inline const patterns (a removed feature) are suddenly syntactically valid / accepted pre expansion. That's not right. For context, below are the errors that stable emits:

error: const blocks cannot be used as patterns
 --> file.rs:4:16
  |
4 | analyze!(const { 0 });
  |                ^^^^^
  |
  = help: use a named `const`-item or an `if`-guard (`x if x == const { ... }`) instead

error: const blocks cannot be used as patterns
 --> file.rs:2:24
  |
2 | fn scope() { let const { 0 }; }
  |                        ^^^^^
  |
  = help: use a named `const`-item or an `if`-guard (`x if x == const { ... }`) instead

They're now parsed as "Pat(Expr(ConstBlock(AnonConst(..))))" (with mgca_disambiguation AnonConst) whereas before they were rejected+recovered as "Pat(Expr(Err(ErrorGuaranteed)))".

The bisection points to nightly-2025-12-30. Right, the correctness regression happened in PR #149667.

rustc 1.94.0-beta.2 (23a44d3c7 2026-01-25)
binary: rustc
commit-hash: 23a44d3c70448c08dc6a2fc13c1afceab49f2bb9
commit-date: 2026-01-25
host: x86_64-unknown-linux-gnu
release: 1.94.0-beta.2
LLVM version: 21.1.8

rustc 1.95.0-nightly (6efa357bf 2026-02-08)
binary: rustc
commit-hash: 6efa357bff60d192688e02de0c78cae24a7f3a55
commit-date: 2026-02-08
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 22.1.0

[fmease]

cc @dianne @Shinonn23 (#149667)