nvcc 9.0 emits spurious warnings regarding ignored annotations

[jaredhoberock]

When compiling Agency program, nvcc 9.0 emits warnings of this form:

agency/detail/tuple/tuple_leaf.hpp(59): warning: __host__ annotation on a defaulted function("tuple_leaf_base") is ignored

Unfortunately, the issue noted in nvbug 1966948 prevents a workaround, so these warnings cannot be eliminated except by downgrading to nvcc 8.

Fortunately, these warnings are benign and can be ignored until a fix is implemented in a newer nvcc.