CUDA: Fix incorrect target inference for implicit members.

As PR20495 demonstrates, Clang currenlty infers the CUDA target (host/device,
etc) for implicit members (constructors, etc.) incorrectly. This causes errors
and even assertions in Clang when compiling code (assertions in C++11 mode where
implicit move constructors are added into the mix).

Fix the problem by inferring the target from the methods the implicit member
should call (depending on its base classes and fields).