[CUDA] Make unattributed constexpr functions implicitly host+device.

[CUDA] Make unattributed constexpr functions implicitly host+device.

With this patch, by a constexpr function is implicitly host+device
unless:

 a) it's a variadic function (variadic functions are not allowed on the
    device side), or
 b) it's preceeded by a __device__ overload in a system header.

The restriction on overloading __host__ __device__ functions on the
basis of their CUDA attributes remains in place, but we use (b) to allow
us to define __device__ overloads for constexpr functions in cmath,
which would otherwise be __host__ __device__ and thus not overloadable.

You can disable this behavior with -fno-cuda-host-device-constexpr.

Reviewers: tra, rnk, rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18380

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@264964 91177308-0d34-0410-b5e6-96231b3b80d8