[mlir][Linalg] Allow comprehensive bufferization to use callbacks for alloc/dealloc.

Using callbacks for allocation/deallocation allows users to override
the default.
Also add an option to comprehensive bufferization pass to use `alloca`
instead of `alloc`s. Note that this option is just for testing. The
option to use `alloca` does not work well with the option to allow for
returning memrefs.