[OPENMP]Check that global vars require predefined allocator.

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@356496 91177308-0d34-0410-b5e6-96231b3b80d8