[flang][runtime] Add dynamically allocated temporary storage

These functions will be used as part of the HLFIR lowering for
forall/where. The contents of the API were requested by @jeanPerier.

The API is designed around that use case, assuming that the caller knows
through some side channel what size to allocate for boxes returned from
the pop() function.

Differential Revision: https://reviews.llvm.org/D150050