[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

llvm-svn: 267811