[CUDA] Don't define __NVCC__.

Summary:
We decided this makes life too difficult for code authors.  For example,
people may want to detect NVCC and disable variadic templates, which
NVCC does not support, but which we do.

Since people are going to have to change compiler flags *anyway* in
order to compile with clang, if they really want the old behavior, they
can pass -D__NVCC__.

Tested with tensorflow and thrust, no apparent problems.

Reviewers: tra

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18417

llvm-svn: 264205