Replacing ICC C++14 with C++17 (#3570)

* Replacing ICC C++14 with C++17

* ICPC: -diag-disable:conversion

Try to suppress the `-Werror-all` promotion in `#include <variant>`

* Trying `-DPYBIND11_WERROR=OFF`

* Trying `-Wno-conversion` (todo: make specific to C++17 if it works)

* Inserting -Wno-conversion only for C++17

Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>