Support C++20 language features

Qt Creator should be able to correctly parse code that use latest C++20, like the spaceship operator:

#include <compare>
struct MyInt {
  int value;
  MyInt(int value): value{value} { }
  auto operator<=>(const MyInt&) const = default;
};

Currently results in

error: 'operator<=' cannot be the name of a variable or data member

(TODO: We should be able to find a repo with some testbed for C++ 20 language features)