[CUDA] Add __declspec spellings for CUDA attributes.

Summary: CUDA attributes are spelled __declspec(__foo__) on Windows.

Reviewers: tra

Subscribers: cfe-commits, rnk

Differential Revision: https://reviews.llvm.org/D28321

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@291134 91177308-0d34-0410-b5e6-96231b3b80d8