Features: Add special case to disable relaxed constexpr for Intel 18
Intel compilers define `__cpp_constexpr` to `200704` even in C++14 mode. This indicates that the `cxx_relaxed_constexpr` feature is not available. However, Intel 17 and above document support for it. In commit v3.8.0~9^2 (Features: Update features for Intel C++ 17.0.2 on UNIX, 2017-03-31) we added a special check for this case. Intel 17 and 19 work. However, Intel 18 does not seem to work and fails our test case. Add a special case to disable the feature for Intel 18.