gen_stub: support use statements in stub files

Stmt\Use_ and Stmt\GroupUse nodes were not handled in handleStatements(),
causing an "Unexpected node" exception when use statements appeared in
stub files. Since NameResolver resolves all names to their fully qualified
form before handleStatements() runs, these nodes can simply be skipped.

Author: GromNaN

build/gen_stub.php

@@ -4387,6 +4387,11 @@ private function handleStatements(array $stmts, PrettyPrinterAbstract $prettyPri
                 }
             }
 
+            if ($stmt instanceof Stmt\Use_ || $stmt instanceof Stmt\GroupUse) {
+                // use statements are resolved by NameResolver before this point
+                continue;
+            }
+
             throw new Exception("Unexpected node {$stmt->getType()}");
         }
         if (!empty($conds)) {