[OpenMP] Refactor code that calls codegen for target regions on the device.

This patch refactors code that calls codegen for target regions.  Currently
the codebase only supports the 'target' directive.  The patch pulls out
common target processing code into a static function that can be called
by codegen for any target directive.

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D28752

llvm-svn: 292134