[RFC][flang] Add support for assumed-shape dummy arrays repacking. (#127147)

This is a document describing why and how to add support for repacking
of assumed-shape dummy arrrays to provide more efficient data cache.
It proposes adding new FIR operations and outlines the compiler flow
handling these operations.
I would like to hear feedback on all of it, but especially on:
  * The possibility of detecting safeness of the repacking
    in the context of OpenACC/OpenMP. If it is not possible
    to do the runtime checks to determine safety, then
    there is not need to add the `TempCopyIsSafe` attributes
    to the instruction.
  * Whether it is possible to preserve the debug information
    in cases where `fir.pack_array` is sunk after `[hl]fir.declare`,
    so that before the `fir.pack_array` a debugger will refer
    to the values in the original array, and after `fir.pack_array`
    it will refer to the copy.

Author: vzakhari