[OpenMP] Code generation for target exit data directive

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

Given that, apart from the employed runtime call, target exit data requires the same code generation pattern as target enter data, the OpenMP codegen entry point was renamed and reused for both.

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

Subscribers: cfe-commits, fraggamuffin, caomhin

Differential Revision: http://reviews.llvm.org/D17369

llvm-svn: 267814