[flang][OpenACC] add pass to bufferize fir.box recipes (#163543)

When working on privatization, it is easier to work with fir.box
explicitly in memory, otherwise, there is no way to express that the
fir.box will end-up being a descriptor address in FIR which makes it
hard to deal with data management.

However, introducing fir.ref<fir.box> early can pessimize early HLFIR
optimization because it is harder to reason about the aliasing of
`fir.ref<fir.box>` because of the extra memory indirection.

This patch introduces a pass that turns acc `!fir.box<T>` recipes into
`!fir.ref<!fir.box<T>>` recipes and updated the related recipe usages to
use `!fir.ref<!fir.box<T>>` (creating new alloca+store+load).

It is added to flang and not OpenACC because it is specific to the
`fir.box` type, so it makes little sense to make it an OpenACC generic
pass and to create a new OpenACC dialect type interface for this use
case.

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