Brought up in a previous review as a TODO, we could be better about how
we highlight what hte previous clause was, and how to show that the
'device_type' is the one being targetted. This patch rewords the
diagnostics and updates a massive number of tests.
Similar to 'worker', the 'vector' clause has some rules that needed to
be applied on its argument legality that for combined constructs need to
look at the current construct, not the 'effective' parent construct.
Additionally, it has some interaction with `vector_length` that needed
to be encoded as well. This patch implements it.
