[OpenMP] Codegen support for 'target teams' on the NVPTX device.

This is a simple patch to teach OpenMP codegen to emit the construct
in Generic mode.

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D29143


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293183 91177308-0d34-0410-b5e6-96231b3b80d8