[CUDA] Normalize handling of defauled dtor.

Defaulted destructor was treated inconsistently, compared to other
compiler-generated functions.

When Sema::IdentifyCUDATarget() got called on just-created dtor which didn't
have implicit __host__ __device__ attributes applied yet, it would treat it as a
host function.  That happened to (sometimes) hide the error when dtor referred
to a host-only functions.

Even when we had identified defaulted dtor as a HD function, we still treated it
inconsistently during selection of usual deallocators, where we did not allow
referring to wrong-side functions, while it is allowed for other HD functions.

This change brings handling of defaulted dtors in line with other HD functions.

Differential Revision: https://reviews.llvm.org/D94732