[CUDA] Work around a new quirk in CUDA9 headers.

In CUDA-9 some of device-side math functions that we need are conditionally
defined within '#if _GLIBCXX_MATH_H'. We need to temporarily undo the guard
around inclusion of math_functions.hpp.

Differential Revision: https://reviews.llvm.org/D37906

llvm-svn: 313369