Make move CTFEable for simple structs and classes

memcpy is not CTFEable, so avoid it for structs without
dtor, postblit that can be assigned with plain =.

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.
Merge pull request #2036 from blackwhale/ctfe-move

Make move CTFEable for simple structs and classes