[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


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293049 91177308-0d34-0410-b5e6-96231b3b80d8