[ThinLTO] Record calls to aliases

Summary:
When there is a call to an alias in the same module, we were not
adding a call edge. So we could incorrectly think that the alias
was dead if it was inlined in that function, despite having a
reference imported elsewhere. This resulted in unsats at link time.

Add a call edge when the call is to an alias.

Reviewers: davide, mehdi_amini

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25384

llvm-svn: 283664