[OpenMP] Add support to map member expressions with references to pointers.

Summary: This patch add support to map pointers through references in class members. Although a reference does not have storage that a user can access, it still has to be mapped in order to get the deep copy right and the dereferencing code work properly.

Reviewers: hfinkel, carlo.bertolli, arpith-jacob, kkwli0, ABataev

Subscribers: caomhin, cfe-commits

Differential Revision: https://reviews.llvm.org/D22787

llvm-svn: 276934