[IR] [DAE] Copy comdats during DAE, and don't copy comdats in GlobalObject::copyAttributesFrom.

Summary: This reverts the changes to Globals.cpp and IRMover.cpp in
"[IR] Copy comdats in GlobalObject::copyAttributesFrom" (D20631,
rL270743).

The DeadArgElim test is left unchanged, and we change DAE to explicitly
copy comdats.

The reverted change breaks copyAttributesFrom when the destination lives
in a different module from the source.  The decision in D21255 was to
revert this patch and handle comdat copying separately from
copyAttributesFrom.

Reviewers: majnemer, rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D21403

llvm-svn: 272855