[MLIR][OpenMP] Normalize lowering of omp.loop_nest

This patch refactors the translation of `omp.loop_nest` operations into LLVM IR
so that it is handled similarly to other operations. Before this change, the
responsibility of translating the loop nest fell into each loop wrapper,
causing code duplication. This patch centralizes that handling of the loop.
One consequence of this was fixing an issue lowering non-inclusive `omp.simd`
loops.

As a result, it is now expected that the handling of composite constructs is
performed collaboratively among translating functions for each operation
involved. At the moment, only `do/for simd` is supported by ignoring SIMD
information, and this behavior is preserved.

The translation of loop wrapper operations need access to the
`llvm::CanonicalLoopInfo` loop information structure in order to apply
transformations to it. This is now created in the nested call to
`convertOmpLoopNest`, so it needs to be passed up to all associated loop
wrapper translation functions. This is done via the creation of an
`OpenMPLoopInfoStackFrame` within `convertOmpLoopNest` and its removal after
its outermost associated loop wrapper has been translated.

Author: skatrak