Revert r313157 "ThinLTO: Correctly follow aliasee references when dead stripping."

Revert r313157 "ThinLTO: Correctly follow aliasee references when dead stripping."

This broke Chromium's CFI build; see crbug.com/765004.

> We were previously handling aliases during dead stripping by adding
> the aliased global's "original name" GUID to the worklist. This will
> lead to incorrect behaviour if the global has local linkage because
> the original name GUID will not correspond to the global's GUID in
> the summary.
>
> Because an alias is just another name for the global that it
> references, there is no need to mark the referenced global as used,
> or to follow references from any other copies of the global. So all
> we need to do is to follow references from the aliasee's summary
> instead of the alias.
>
> Differential Revision: https://reviews.llvm.org/D37789

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313222 91177308-0d34-0410-b5e6-96231b3b80d8