fix(struct): 修复 _Undefined 检查中的未定义变量 constant

列表初始化器中应检查元素 c.constant，原代码引用裸名 constant 导致 NameError。

lib/core/LLVMCG/StructGen.py

@@ -942,7 +942,7 @@ class StructGenMixin:
                 needs_fix = True
             elif isinstance(gv.initializer, ir.Constant):
                 if isinstance(gv.initializer.constant, list):
-                    if any(hasattr(c, 'constant') and isinstance(constant, _Undefined) for c in gv.initializer.constant):
+                    if any(hasattr(c, 'constant') and isinstance(c.constant, _Undefined) for c in gv.initializer.constant):
                         needs_fix = True
             if not needs_fix:
                 continue