Handle `AbstractQ` in concatenation (#51132)
Handling `AbstractQ`s in concatenation got lost in the overhaul. This
brings it back (though it seemed to not be used anywhere), and even
better: pre-1.9 `Q`s where handled via `AbstractMatrix` fallbacks, so
elementwise. Now, it first materializes the matrix, and then copies to
the right place in the destination array.
