[ThinLTO] Indirect call promotion fixes for promoted local functions

Summary:
Fix a couple issues limiting the application of indirect call promotion
in ThinLTO mode:
- Invoke indirect call promotion before globalopt, since it may
  eliminate imported functions which appear unreferenced.
- Invoke indirect call promotion with InLTO=true so that the PGOFuncName
  metadata is used to get the name for locals which would have been
  renamed during promotion.

Reviewers: davidxl, mehdi_amini

Subscribers: Prazek, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D24004

llvm-svn: 280024