[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

llvm-svn: 272854