[Bugpoint] Erase comdat annotations after removing a global's initializer.

[Bugpoint] Erase comdat annotations after removing a global's initializer.

Summary:
This is necessary to keep the verifier happy after bugpoint removes an
initializer from a global variable with a comdat annotation, because
globals without initializers may not have comdats.

Reviewers: majnemer, rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D21274

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272854 91177308-0d34-0410-b5e6-96231b3b80d8