[OpenMP] Add task alloc function

Summary: Add the target task allocation function to the interface.

Reviewers: ABataev, AlexEichenberger, caomhin, jlpeyton, AndreyChurbanov, RaviNarayanaswamy, hbae

Reviewed By: AlexEichenberger, hbae

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

Tags: #openmp, #clang

Differential Revision: https://reviews.llvm.org/D63010

llvm-svn: 363449