[Pipelines] Don't skip GlobalDCE in ThinLTO pre-link

GlobalDCE will only remove functions with available externally
linkage if they are unreferenced. As such, I don't believe there
is any problem with running this pass as part of the ThinLTO pre-link
pipeline. It will only remove functions that are completely dead in
that module, and I don't think there is any benefit to keeping them
around for the post-link phase.

There is no compile-time impact from the additional pass.

This is a followup to one of the side discussions in D146776.

Differential Revision: https://reviews.llvm.org/D149446