Use references now that it is natural to do so.

The linker never takes ownership of a module or changes which module it
is refering to, making it natural to use references.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254449 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed