[OpenMP] Code generation for teams - kernel launching

Summary:
This patch implements the launching of a target region in the presence of a nested teams region, i.e calls tgt_target_teams with the required arguments gathered from the enclosed teams directive.

The actual codegen of the region enclosed by the teams construct will be contributed in a separate patch.

Reviewers: hfinkel, arpith-jacob, kkwli0, carlo.bertolli, ABataev

Subscribers: cfe-commits, caomhin, fraggamuffin

Differential Revision: http://reviews.llvm.org/D17019

llvm-svn: 262625