[OpenMP] Add support for mapping array sections through pointer references.

Summary:
This patch fixes a bug in the map of array sections whose base is a reference to a pointer. The existing mapping support was not prepared to deal with it, causing the compiler to crash.

Mapping a reference to a pointer enjoys the same characteristics of a regular pointer, i.e., it is passed by value. Therefore, the reference has to be materialized in the target region.

Reviewers: hfinkel, carlo.bertolli, kkwli0, ABataev

Subscribers: caomhin, cfe-commits

Differential Revision: https://reviews.llvm.org/D22690

llvm-svn: 276933