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

This patch adds support for the Spmd construct 'target parallel' on the
NVPTX device. This involves ignoring the num_threads clause on the device
since the number of threads in this combined construct is already set on
the host through the call to __tgt_target_teams().

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D29083

llvm-svn: 292999