Reland r313157, "ThinLTO: Correctly follow aliasee references when dead stripping." which was reverted in r313222.

This reland includes a fix for the LowerTypeTests pass so that it
looks past aliases when determining which type identifiers are live.

Differential Revision: https://reviews.llvm.org/D37842

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313229 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed