[OpenMP] Teams reduction on the NVPTX device.

This patch implements codegen for the reduction clause on
any teams construct for elementary data types.  It builds
on parallel reductions on the GPU.  Subsequently,
the team master writes to a unique location in a global
memory scratchpad.  The last team to do so loads and
reduces this array to calculate the final result.

This patch emits two helper functions that are used by
the OpenMP runtime on the GPU to perform reductions across
teams.

Patch by Tian Jin in collaboration with Arpith Jacob

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D29879

llvm-svn: 295335