[OPENMP50]Support for declare variant directive for NVPTX target.

NVPTX does not support global aliases. Instead, we have to copy the full
body of the variant function for the original function.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374387 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed