Make move CTFEable for simple structs and classes

memcpy is not CTFEable, so avoid it for structs without
dtor, postblit that also can be assigned with plain `lhs = rhs;`.

This leaves as is (compile error in CTFE) structs that contain
immutable/const memebers the fact that moves overwrites such
fields is questionable to begin with.

Remark: pointsTo doesn't work at CTFE. The check itself makes no sense for class instances and primitive types.