[CUDA] Mark __libcpp_{isnan,isinf,isfinite} as constexpr.

Summary:
This makes these functions available on host and device, which is
necessary to compile <complex> for the device.

Reviewers: hfinkel, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25403

llvm-svn: 287012