[OpenMP] Support for the proc_bind-clause on 'target parallel' on the NVPTX device.

This patch adds support for the proc_bind clause on the Spmd construct
'target parallel' on the NVPTX device.  Since the parallel region is created
upon kernel launch, this clause can be safely ignored on the NVPTX device at
codegen time for level 0 parallelism.

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D29128

llvm-svn: 293069