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.
This one is a bit complicated, as it has some interesting interactions,
as 'gang' Sema is required to look at its containing compute construct.
Except in the case of a combined construct, they are the same. This
resulted in a large refactor of the checking code for CheckGangExpr,
plus some additional work on the diagnostics for its interaction with
'num_gangs' and 'vector'/'worker'.
