[ThinLTO/WPD] Enable -wholeprogramdevirt-skip in ThinLTO backends

Previously this option could be used to skip devirtualizations of the
given functions in regular LTO and in the ThinLTO indexing step. This
change allows them to be skipped in the backend as well, which is useful
when debugging WPD in a distributed ThinLTO backend.

Differential Revision: https://reviews.llvm.org/D91812