[OpenMP] Try to embed offloading objects after codegen

Currently we use the `-fembed-offload-object` option to embed a binary
file into the host as a named section. This is currently only used as a
codegen action, meaning we only handle this option correctly when the
input is a bitcode file. This patch adds the same handling to embed an
offloading object after we complete code generation. This allows us to
embed the object correctly if the input file is source or bitcode.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D120270