C++20 support in OpenCL

What are the plans to add support for C++20 in C++ for OpenCL? I understand C++20 is huge and many features don't make sense in device code. But a subset of C++20 could still be beneficial. For our use case, we would be particularly interested in compile time evaluation with the `consteval` keyword.

Start by reviewing the C++ for OpenCL documentation and the requested C++20 feature, particularly compile-time evaluation with `consteval`. The issue does not name files, tests, or an entry point. Done would require a defined subset of C++20 support or a clear implementation plan.

Written by the indexing model from the issue text.