[OpenMP] Embed device files into the host IR

This patch adds support for embedding the device object files into the
host IR to create a fat binary. Each offloading file will be inserted
into a section with the following naming format
`.llvm.offloading.<triple>.<arch>.<filename>`.

Depends on D116542

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D116543