[OPENMP]Check that global vars require predefined allocator.

According to OpenMP, 2.11.3 allocate Directive, Restrictions, C / C++,
if a list item has a static storage type, the allocator expression in
  the allocator clause must be a constant expression that evaluates to
  one of the predefined memory allocator values. Added check for this
  restriction.

llvm-svn: 356496