[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276934
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