[region-isolation] Fix verifier check.
After following up with @slavapestov and @xedin, I was right to be suspicious of
my changes here.
Instead of attempting to hard code this, I do the right thing and I map the
relevant type into the function's generic context and then do the check. This
ensures that when isAnyActorType() performs the conformance check,
ModuleDecl::lookupConformance() has a generic signature to work with.
