refactor(argparse): 将null替换为nullptr以提高C++兼容性

- 在argparse库中将所有null指针常量替换为nullptr
- 更新头文件和源文件中的指针初始化和比较操作
- 修改测试文件中的相关断言检查
- 更新AST定义文件中的注释说明

libs/ir/src/scc_ir.c

@@ -1,9 +1,9 @@
 #include <scc_ir.h>
 
 void scc_ir_type_init(scc_ir_type_t *in, scc_ir_type_tag_t tag) {
-    Assert(in != null);
+    Assert(in != nullptr);
     in->tag = tag;
-    in->name = null;
+    in->name = nullptr;
     switch (tag) {
     case SCC_IR_TYPE_unknown:
     case SCC_IR_TYPE_void:
@@ -40,31 +40,33 @@ void scc_ir_type_init(scc_ir_type_t *in, scc_ir_type_tag_t tag) {
 }
 
 void scc_ir_bblock_init(scc_ir_bblock_t *in, const char *label) {
-    Assert(in != null);
-    Assert(label != null);
+    Assert(in != nullptr);
+    Assert(label != nullptr);
     in->label = label;
     scc_vec_init(in->instrs);
 }
 
 void scc_ir_func_init(scc_ir_func_t *in, const char *name) {
-    Assert(in != null);
-    Assert(name != null);
+    Assert(in != nullptr);
+    Assert(name != nullptr);
     in->name = name;
     in->type = 0;
     scc_vec_init(in->bblocks);
     scc_vec_init(in->params);
 }
 
-void scc_ir_node_init(scc_ir_value_t *in, const char *name,
-                      scc_ir_value_tag_t tag) {
-    Assert(in != null);
+void scc_ir_value_init(scc_ir_value_t *in, const char *name,
+                       scc_ir_value_tag_t tag) {
+    Assert(in != nullptr);
     in->name = name;
     in->tag = tag;
     scc_vec_init(in->used_by);
     in->type = 0;
 
     switch (tag) {
-    case SCC_IR_VALUE_TAG_NULL:
+    case SCC_IR_VALUE_TAG_NULLPTR:
+        break;
+    case SCC_IR_VALUE_TAG_BUILTIN:
         break;
     case SCC_IR_VALUE_TAG_CONST_INT:
         // TODO