[OpenMP] Codegen for use_device_ptr clause.

Summary: This patch adds support for the use_device_ptr clause. It includes changes in SEMA that could not be tested without codegen, namely, the use of the first private logic and mappable expressions support.

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

Subscribers: caomhin, cfe-commits

Differential Revision: https://reviews.llvm.org/D22691

llvm-svn: 276977