Fix: BlockExpr translation

Fix after breaking change in liblktlang.

See:
https://gitlab.adacore-it.com/eng/libadalang/langkit/-/merge_requests/1404

Author: Killian Perlin

lkql_jit/language/src/main/java/com/adacore/lkql_jit/langkit_translator/passes/LktPasses.java

@@ -353,7 +353,8 @@ private Expr buildExpr(Liblktlang.Expr expr) {
                     final var clause = (Liblktlang.BlockExprClause) blockExpr
                         .fClauses()
                         .getChild(i);
-                    final var valDecl = (Liblktlang.ValDecl) clause.fClause();
+                    final var valDecl =
+                        (Liblktlang.ValDecl) ((Liblktlang.FullDecl) clause.fClause()).fDecl();
                     blockBody[i] = new BlockBodyDecl(loc(valDecl), buildDecl(valDecl));
                 }
                 final var subExpr = buildExpr(