Correct type of static forms in hsExprType

The simplest way to do this seemed to be to persist the whole type in
the extension field from the typechecker so that the few relevant places

 * Desugaring can work out the return type by splitting this type rather
   than calling `dsExpr` (slightly more efficient).
 * hsExprType can just return the correct type.
 * Zonking has to now zonk the type as well

The other option we considered was wiring in StaticPtr but that is
actually quite tricky because StaticPtr refers to StaticPtrInfo which
has field selectors (which we can't easily wire in).

Fixes #20150