[OpenMP] Add stubs for Task affinity API

This patch adds __kmpc_omp_reg_task_with_affinity to register affinity
information for tasks. For now, the affinity information is not used,
and the function always succeeds. This also adds the kmp_task_affinity_info_t
structure to store the task affinity information.

Patch by Terry Wilmarth

Differential Revision: https://reviews.llvm.org/D55026

llvm-svn: 347907