Rename variables to reflect linker split (NFC)

Renamed variables to be more reflective of whether they are
an instance of Linker, IRLinker or ModuleLinker. Also fix a stale
comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256011 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed