[OpenMP] Code generation for target enter data directive

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

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

Subscribers: cfe-commits, fraggamuffin, caomhin

Differential Revision: http://reviews.llvm.org/D17368

llvm-svn: 267812