refactor(ast): 调整AST结构体和枚举类型的声明表示方式

将结构体、联合和枚举类型的字段表示从向量改为声明指针，
允许name和decl字段为null，更新相关初始化函数的断言检查，
使结构更加灵活并支持不完整类型定义。

BREAKING CHANGE: 修改了scc_ast_type结构体中record和enumeration
子类型的字段表示方法，从fields向量改为decl指针。

libs/parser/src/parse_decl.c

@@ -155,25 +155,6 @@ A.2.4 External definitions
 #include <scc_parser.h>
 
 scc_ast_decl_t *scc_parse_declaration(scc_parser_t *parser) {
-    /**
-     * ISO/IEC 9899:TC3
-     * 6.7 Declarations
-     * Syntax
-     *
-     * declaration:
-     *     declaration-specifiers init-declarator-list(opt) ;
-     * declaration-specifiers:
-     *     storage-class-specifier declaration-specifiers(opt)
-     *     type-specifier declaration-specifiers(opt)
-     *     type-qualifier declaration-specifiers(opt)
-     *     function-specifier declaration-specifiers(opt)
-     * init-declarator-list:
-     *     init-declarator
-     *     init-declarator-list , init-declarator
-     * init-declarator:
-     *     declarator
-     *     declarator = initializer
-     */
     cbool ok;
     scc_lexer_tok_t tok;
 
@@ -184,8 +165,20 @@ scc_ast_decl_t *scc_parse_declaration(scc_parser_t *parser) {
     }
 
     if (SCC_AST_IS_A(scc_ast_type_t, type_or_decl)) {
-        LOG_WARN("declaration dose not declare anything");
-        return null;
+        scc_ast_type_t *type = SCC_AST_CAST_TO(scc_ast_type_t, type_or_decl);
+        if (type->base.type == SCC_AST_TYPE_STRUCT ||
+            type->base.type == SCC_AST_TYPE_UNION) {
+            Assert(type->record.decl != null);
+            decl = type->record.decl;
+            scc_free(type_or_decl); // FIXME
+        } else if (type->base.type == SCC_AST_TYPE_ENUM) {
+            Assert(type->enumeration.decl != null);
+            decl = type->enumeration.decl;
+            scc_free(type_or_decl); // FIXME
+        } else {
+            LOG_WARN("declaration dose not declare anything");
+            return null;
+        }
     } else if (SCC_AST_IS_A(scc_ast_decl_t, type_or_decl)) {
         decl = SCC_AST_CAST_TO(scc_ast_decl_t, type_or_decl);
     } else {