[flang][runtime] Added Fortran::common::optional for use on device.

This is a simplified implementation of std::optional that can be used
in the offload builds for the device code. The methods are properly
marked with RT_API_ATTRS so that the device compilation succedes.

Reviewers: klausler, jeanPerier

Reviewed By: jeanPerier

Pull Request: https://github.com/llvm/llvm-project/pull/85177