[Driver][OpenMP] Update actions builder to create unbundling action when necessary.

Summary:
Each time that offloading support is requested by the user and the input file is not a source file, an action `OffloadUnbundlingAction` is created to signal that the input file may contain bundles, so that the proper tool is then invoked to attempt to extract the components of the bundle. This patch adds the logic to create that action in offload action builder.

The job creation for the new action will be proposed in a separate patch.

Reviewers: echristo, tra, jlebar, ABataev, hfinkel

Subscribers: whchung, mehdi_amini, cfe-commits, Hahnfeld, andreybokhanko, arpith-jacob, carlo.bertolli, caomhin

Differential Revision: https://reviews.llvm.org/D21853

llvm-svn: 285324