Given we optimize for the case where the new value is of the
same type of the one already stored in the variant, enable move
assignment for that case.
As a drive-by, avoid a path to detach() for data() if we know
we're detached.
Change-Id: I9abbdc10637ce77ebb747b49d83e1ef914d997bb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>