Fix up C++ version detection macros and cmake tests.

Eigen was reporting the wrong c++ version for intermediate versions of __cplusplus. Also disabling explicit c++17/c++20 constexpr tests, since these are breaking on our CI. It looks like some versions of clang are reporting __cplusplus as 20, but don't support all c++20 features.

CI failures: https://gitlab.com/libeigen/eigen_ci_cross_testing/-/pipelines/725775539

Fixes #2584 (closed)