LTO: -internalize sets visibility to default

Visibility is meaningless when the linkage is local.  Change
`-internalize` to reset the visibility to `default`.

<rdar://problem/16141113>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207979 91177308-0d34-0410-b5e6-96231b3b80d8