[OpenMP] Codegen for target update directive.

Summary: This patch implements the code generation for the `target update` directive. The implemntation relies on the logic already in place for target data standalone directives, i.e. target enter/exit data.

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

Subscribers: caomhin, cfe-commits

Differential Revision: http://reviews.llvm.org/D20650

llvm-svn: 270886