[OpenMP] Code generation for the is_device_ptr clause

Summary: This patch adds support for the is_device_ptr clause. It expands SEMA to use the mappable expression logic that can only be tested with code generation in place and check conflicts with other data sharing related clauses using the mappable expressions infrastructure.

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

Subscribers: caomhin, cfe-commits

Differential Revision: https://reviews.llvm.org/D22788

llvm-svn: 276978