Intel: Fix default C++ dialect detection on Windows
For the Intel Compiler for Windows we have some subtle preprocessor checks in compiler feature detection to detect C++11 and C++14 modes. Use these when detecting the default C++ dialect too.
Also update the
CompileFeatures test for Intel 19 with VS 2015. Blacklist
cxx_relaxed_constexpr for this case because the compiler does not advertise the support in this mode but compiles our example.