This reverts commit d310959.

…CLARE_HOLDER_TYPE()`

… rwgk why it does not work).

This is the beginning of the error message:

```
         D:\a\pybind11\pybind11\tests\test_smart_ptr.cpp(285,1): error C2143: syntax error: missing ')' before ',' [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
         D:\a\pybind11\pybind11\tests\test_smart_ptr.cpp(285,1): error C2059: syntax error: ')' [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
```

…nts")` in test_smart_ptr.cpp

This is the error message:

```
/__w/pybind11/pybind11/tests/test_smart_ptr.cpp:287:51: error: must specify at least one argument for '...' parameter of variadic macro [-Werror,-Wgnu-zero-variadic-macro-arguments]
PYBIND11_DECLARE_HOLDER_TYPE(T, std::shared_ptr<T>)
                                                  ^
/__w/pybind11/pybind11/include/pybind11/cast.h:885:13: note: macro 'PYBIND11_DECLARE_HOLDER_TYPE' defined here
            ^
```

…rguments")` in test_virtual_functions.cpp

…rguments")` in test_embed/test_interpreter.cpp

…uments")` near the top of pybind11/pybind11.h; 2. Change `PYBIND11_DECLARE_HOLDER_TYPE` macro to side-step the only remaining clang warning-as-error (this is still needed even for clang 18).

Alternatively the warning suppression could be moved into pybind11/cast.h, but this commit limits the warning suppression to smaller scope within include/pybind11.