[CUDA] Declare our __device__ math functions in the same inline namespace as our standard library.

Summary:
Currently we declare our inline __device__ math functions in namespace
std.  But libstdc++ and libc++ declare these functions in an inline
namespace inside namespace std.  We need to match this because, in a
later patch, we want to get e.g. <complex> to use our device overloads,
and it only will if those overloads are in the right inline namespace.

Reviewers: tra

Subscribers: cfe-commits, jhen

Differential Revision: https://reviews.llvm.org/D24977

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283678 91177308-0d34-0410-b5e6-96231b3b80d8