[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270886
91177308-0d34-0410-b5e6-
96231b3b80d8