[flang][openacc] Support assumed shape arrays in reduction (#67610)

Assumed shape array are using descriptor and must be handled differently
than known shape arrays. This patch adds support to generate the `init`
and `combiner` region for the reduction recipe operation with assumed
shape array by using the descriptor and the HLFIR lowering path.

`createTempFromMold` function is moved from
`flang/lib/Optimizer/HLFIR/Transforms/BufferizeHLFIR.cpp` to
`flang/include/flang/Optimizer/Builder/HLFIRTools.h` to be reused to
create the private copy.