CompileFeatures: Relax cxx_relaxed_constexpr compiler requirements

This in effect means that cxx_relaxed_constexpr is now supported
by MSVC and Intel 18.0-18.04.

Tests/CompileFeatures/cxx_relaxed_constexpr.cpp

@@ -22,6 +22,7 @@ constexpr int g(const int (&is)[4])
 
 int someFunc()
 {
-  constexpr int k3 = g({ 4, 5, 6, 7 });
+  constexpr int values[4] = { 4, 5, 6, 7 };
+  constexpr int k3 = g(values);
   return k3 - 42;
 }