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
!652 (merged) 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.