[ThinLTO] Look through aliases when computing hash keys

Without this, we don't consider types used by aliasees in our cache key.
This caused issues when using the same cache for thin-linking the same
TU with different sets of virtual call candidates for a virtual call
inside of a constructor. That's sort of a mouthful. :)

Differential Revision: https://reviews.llvm.org/D55060

llvm-svn: 348216