[OPENMP]Consider 'omp_null_allocator' as a predefined allocator.

Summary:
omp.h header file defines omp_null_allocator as a predefined allocator,
need to consider it also as a predefined allocator.

Reviewers: jdoerfert

Subscribers: jholewinski, yaxunl, guansong, cfe-commits, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79186