[IR] Copy comdats in GlobalObject::copyAttributesFrom

This is probably correct for all uses except cross-module IR linking,
where we need to move the comdat from the source module to the
destination module.

Fixes PR27870.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D20631

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270743 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed