[OpenMP] Allow negative lower bound in array sections based on pointers

OpenMP 4.5 removed the restriction that array section lower bound must be non negative.
This change is to allow negative values for array section based on pointers.
For array section based on array type there is still a restriction: "The array section must be a subset of the original array."

Patch by David S.

Differential Revision: https://reviews.llvm.org/D22481

llvm-svn: 276177