[OpenMP] Support for thread_limit-clause on the 'target teams' directive.

The thread_limit-clause on the combined directive applies to the
'teams' region of this construct. We modify the ThreadLimitClause
class to capture the clause expression within the 'target' region.

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D29087

llvm-svn: 293049