* Removing C++20 condition for MSVC is_template_base_of decltype workaround.

* `-DDOWNLOAD_EIGEN=ON` for MSVC 2022 C++20

* `-DDOWNLOAD_EIGEN=ON` for MSVC 2019 C++20

* `-DPYBIND11_WERROR=OFF` for MSVC C++20 (2019, 2020)

* Restoring `defined(PYBIND11_CPP20)` in common.h

* pragma warning(disable : 5054) in eigen.h

* Reverting `-DPYBIND11_WERROR=OFF` changes.