[OPENMP]Improve detection of omp_allocator_handle_t type and predefined

allocators.

It is better to deduce omp_allocator_handle_t type from the predefined
allocators, because omp.h header might not define it explicitly. Plus,
it allows to identify the predefined allocators correctly when trying to
build the allcoator for the global variables.

llvm-svn: 356607