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

The num_threads-clause on the combined directive applies to the
'parallel' region of this construct. We modify the NumThreadsClause
class to capture the clause expression within the 'target' region.

The offload runtime call for 'target parallel' is changed to
__tgt_target_teams() with 1 team and the number of threads set by
this clause or a default if none.

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D29082

llvm-svn: 292997