Nvcc emits warnings regarding standard library functions calling __host__-only functions

[jaredhoberock]

For example:

test.cu(12): warning: calling a __host__ function("std::__shared_count<( ::__gnu_cxx::_Lock_policy)2> ::~__shared_count") from a __host__ __device__ function("std::future<void> ::~future") is not allowed

std::future::~future happens to be a compiler-generated function and therefore its execution space specifiers are inferred by the compiler. Some of Agency's templated __host__ __device__ functions may create and destroy std::futures. As a result, std::future::~future is inferred to be __host__ __device__. The problem is that implementations of std::future call functions like the above which are not inferred to be __host__ __device__