[llvm] [ThinLTO] Fix parsing null aliasee in alias summary (PR #169490)

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson approved this pull request.

lgtm once the tests pass.

Also want to request for future PRs that force pushes be avoided if at all possible, since it prevents comparing to older commits in github (it's a lot easier to ask github to show me changes since last review, which doesn't work if force pushes occur). See https://llvm.org/docs/GitHub.html#rebasing-pull-requests-and-force-pushes

https://github.com/llvm/llvm-project/pull/169490