-help output cleanup

This is only a start, we could probably go much further in terms of usability by e.g. leveraging different sections. Also, some of the flags that are shown by default are probably of debatable use for most people, e.g. `-v-cg` (who uses this anyway?), `-vv` (I'm thinking about statically disabling the logger in release builds unless it is explicitly enabled in CMake), …

Unfortunately, it is not (easily) possible to implement this on pre-3.3 LLVM versions.