[clang] [llvm] [clang][llvm][fatlto] Avoid cloning modules in FatLTO (PR #72180)

[Paul Kirth via cfe-commits]

ilovepi wrote:

@nikic are there any other changes you'd like to see? otherwise, I plan to rebase and land this later today.

https://github.com/llvm/llvm-project/pull/72180