[LTO] Promote before performing weak resolution

Summary:
This was reversed compared to ThinLTOCodeGenerator for some reason,
and lead to an increased code-size on my tests. I figured that the
weak resolution may internalize a linkonce function, which will be
promoted immediately (and renamed), before being internalized again.

Reviewers: tejohnson

Subscribers: pcc, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D23632

llvm-svn: 279021