[flang][OpenMP] Rewrite `omp.loop` to semantically equivalent ops (#115443)

Introduces a new conversion pass that rewrites `omp.loop` ops to their
semantically equivalent op nests bases on the surr

[flang][OpenMP] Rewrite `omp.loop` to semantically equivalent ops (#115443)

Introduces a new conversion pass that rewrites `omp.loop` ops to their
semantically equivalent op nests bases on the surrounding/binding
context of the `loop` op. Not all forms of `omp.loop` are supported yet.
See `isLoopConversionSupported` for more info on which forms are
supported.

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