[clang] [clang][fatlto] Don't set ThinLTO module flag with FatLTO (PR #75079)

[Teresa Johnson via cfe-commits]

teresajohnson wrote:

Added a comment to that issue, I think it would be good to understand why unified LTO is not expected in that case (for the assertion).

https://github.com/llvm/llvm-project/pull/75079