Remove unneeded struct; NFC

Summary:
Unless I'm missing something, the DeferredGlobal struct's GV field is
unused, removing which makes the struct itself trivial.

Reviewers: rafael, chandlerc

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D32691

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@301789 91177308-0d34-0410-b5e6-96231b3b80d8