IPO/LTO support for ICX (IntelLLVM) compiler (#4402)

* IPO/LTO support for ICX (IntelLLVM) compiler

https://github.com/pybind/pybind11/issues/4080



* style: pre-commit fixes

* Add WARNING/HELP WANTED comment.

Co-authored-by: Martin Blanchard <martin.blanchard@siemens.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>