[clang] [flang] [llvm] Reapply "Introduce -fexperimental-loop-fusion to clang and flang (PR #158844)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

(FWIW, it's helpful to keep the change since last land in a separate commit, otherwise it's hard to tell what exactly changed between the two.)

https://github.com/llvm/llvm-project/pull/158844