[llvm-branch-commits] [LTO] Enable PrepareForLTO in pre-link pipelines (PR #229151)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/229151

This reverts 272ad5da54a63a16312e8c3ba0de4290d4931002. This should not
have any impact on CodeGeneration given PrepareForLTO is now a no-op
currently in LoopUnroll. This just gets the plumbing done for some
future planned uses around respecting pragmas.

This also pulls in changes from #219604 to be complete.