[CUDA] Include CUDA headers before anything else.

Summary:
There's no point to --cuda-path if we then go and include /usr/include
first.  And if you install the right packages, Ubuntu will install (very
old) CUDA headers there.

Reviewers: tra

Subscribers: cfe-commits, Prazek

Differential Revision: https://reviews.llvm.org/D23341

llvm-svn: 278734