Fix ICE by rejecting const blocks in patterns during AST lowering

This fixes the ICE reported by rejecting `const` blocks in
pattern position during AST lowering.

Previously, `ExprKind::ConstBlock` could reach HIR as `PatExprKind::ConstBlock`,
allowing invalid patterns to be type-checked and triggering an ICE.
This patch removes the lowering path for const blocks in patterns
and emits a proper diagnostic instead.

A new UI test is added to ensure the compiler reports a regular error
and to prevent regressions.

clippy_lints/src/utils/author.rs

@@ -723,7 +723,6 @@ macro_rules! kind {
                 kind!("Lit {{ ref {lit}, {negated} }}");
                 self.lit(lit);
             },
-            PatExprKind::ConstBlock(_) => kind!("ConstBlock(_)"),
             PatExprKind::Path(_) => self.maybe_path(pat),
         }
     }

clippy_utils/src/consts.rs

@@ -578,7 +578,6 @@ pub fn eval_pat_expr(&self, pat_expr: &PatExpr<'_>) -> Option<Constant> {
                     Some(val)
                 }
             },
-            PatExprKind::ConstBlock(ConstBlock { body, .. }) => self.expr(self.tcx.hir_body(*body).value),
             PatExprKind::Path(qpath) => self.qpath(qpath, pat_expr.hir_id),
         }
     }

clippy_utils/src/hir_utils.rs

@@ -506,9 +506,8 @@ fn eq_pat_expr(&mut self, left: &PatExpr<'_>, right: &PatExpr<'_>) -> bool {
                     negated: right_neg,
                 },
             ) => left_neg == right_neg && left.node == right.node,
-            (PatExprKind::ConstBlock(left), PatExprKind::ConstBlock(right)) => self.eq_body(left.body, right.body),
             (PatExprKind::Path(left), PatExprKind::Path(right)) => self.eq_qpath(left, right),
-            (PatExprKind::Lit { .. } | PatExprKind::ConstBlock(..) | PatExprKind::Path(..), _) => false,
+            (PatExprKind::Lit { .. } | PatExprKind::Path(..), _) => false,
         }
     }
 
@@ -1102,7 +1101,6 @@ pub fn hash_pat_expr(&mut self, lit: &PatExpr<'_>) {
                 lit.node.hash(&mut self.s);
                 negated.hash(&mut self.s);
             },
-            PatExprKind::ConstBlock(c) => self.hash_body(c.body),
             PatExprKind::Path(qpath) => self.hash_qpath(qpath),
         }
     }