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When importing functions from some module X into some module Y, they may reference other functions already present in Y. The signature (especially attributes) of those other functions may have diverged between X and Y (e.g. due to the Dead Argument Elimination optimization). If necessary, modify the attributes to avoid UB. See the added test and implementation comments for more details. This was exposed by https://reviews.llvm.org/D133036 before it was reverted. Fixes https://github.com/llvm/llvm-project/issues/58976. Reviewed By: tejohnson Differential Revision: https://reviews.llvm.org/D139209