Fix fallout caused by D89156 on 11.0.1 for MacOS
D89156 pluralized the name (old_modules with an s) in all the places it made this change, since it's now a collection. IMO it would make sense to do the same in this change.
Landed in 11.1 https://github.com/llvm/llvm-project/commit/1fdec59bffc11ae37eb51a1b9869f0696bfd5312#diff-2fa23ad0cf1839955ddaf4a0d78a9d9b5fd9b88933f82f6433035916a2655c6c.
Patch needs to be rebased for trunk now.