[Driver][OpenMP] Add support to create jobs for unbundling actions.

Summary:
This patch adds the support to create jobs for the `OffloadBundlingAction` which will invoke the `clang-offload-bundler` tool to unbundle input files.

Unlike other actions, unbundling actions have multiple outputs. Therefore, this patch adds the required changes to have a variant of `Tool::ConstructJob` with multiple outputs. 

The way the naming of the results is implemented is also slightly modified so that the same action can use a different offloading prefix for each use by the different offloading actions. 

With this patch, it is possible to compile a functional OpenMP binary with offloading support, even with separate compilation.

Reviewers: echristo, tra, jlebar, ABataev, hfinkel

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

Differential Revision: https://reviews.llvm.org/D21857

llvm-svn: 285326