[OpenMP] Add target task alloc function with device ID

Summary: Add a new call to Clang to perform task allocation for the target.

Reviewers: ABataev, AlexEichenberger, caomhin

Reviewed By: ABataev, AlexEichenberger

Subscribers: openmp-commits, Hahnfeld, guansong, jdoerfert, cfe-commits

Tags: #clang, #openmp

Differential Revision: https://reviews.llvm.org/D63009

llvm-svn: 363451