If the destination module all ready has a copy of the global coming from the

source module *and* it must be merged (instead of simply replaced or appended
to), then merge instead of replacing or adding another global.

The ObjC __image_info section was being appended to because of this
failure. This caused a crash because the linker expects the image info section
to be a specific size.

<rdar://problem/8198537>

llvm-svn: 115753