Study the clang implementation of metaclasses and the feasibility of using them instead of MOC [Spike]

As per Herb Sutter's presentation in the Spring of 2017 in Oslo, there's a future proposal for new reflection facilities, and metaclasses: http://open-std.org/JTC1/SC22/WG21/docs/papers/2017/p0707r1.pdf

The purpose of this task is to evaluate how we could utilize such a mechanism in Qt's types that today generate code via MOC, and how to make that generation happen with just a C++ compiler.