[mlir][OpenMP] implement SIMD reduction (#146671)

This replicates clang's implementation. Basically:
- A private copy of the reduction variable is created, initialized to
the reduction neutral value (using regions from the reduction
declaration op).
- The body of the loop is lowered as usual, with accesses to the
reduction variable mapped to the private copy.
- After the loop, we inline the reduction region from the declaration op
to combine the privatized variable into the original variable.
- As usual with the SIMD construct, attributes are added to encourage
vectorization of the loop and to assert that memory accesses in the loop
don't alias across iterations.

I have verified that simple scalar examples do vectorize at -O3 and the
tests I could find in the Fujitsu test suite produce correct results. I
tested on top of #146097 and this seemed to work for composite
constructs as well.

Fixes #144290

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