fix(frontend): 补充不支持语法的警告输出

src/irgen/IRGenDecl.cpp

@@ -28,7 +28,7 @@ void IRGenImpl::GenBlock(const ast::Block& block) {
 void IRGenImpl::GenVarDecl(const ast::VarDecl& decl) {
   auto it = locals_.find(decl.name);
   if (it == locals_.end()) {
-    throw std::runtime_error("变量栈槽未创建: " + decl.name);
+    throw std::runtime_error("[irgen] 变量栈槽未创建: " + decl.name);
   }
 
   ir::Value* init = nullptr;