[OpenMP] Code generation for target data directive

[OpenMP] Code generation for target data directive

Summary:
This patch adds support for the target data directive code generation.

Part of the already existent functionality related with data maps is moved to a new function so that it could be reused.

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

Subscribers: cfe-commits, fraggamuffin, caomhin

Differential Revision: http://reviews.llvm.org/D17367

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@267811 91177308-0d34-0410-b5e6-96231b3b80d8