[OpenMP] Initial implementation of parse+sema for clause use_device_ptr of 'target...

[OpenMP] Initial implementation of parse+sema for clause use_device_ptr of 'target data'

http://reviews.llvm.org/D21904

This patch is similar to the implementation of 'private' clause: it adds a list of private pointers to be used within the target data region to store the device pointers returned by the runtime.
Please refer to the following document for a full description of what the runtime witll return in this case (page 10 and 11):
https://github.com/clang-omp/OffloadingDesign

I am happy to answer any question related to the runtime interface to help reviewing this patch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275271 91177308-0d34-0410-b5e6-96231b3b80d8