[CS] Avoid crashing if we have no contextual type for initialization
For `CTP_Initialization`, there's no contextual
type to record if the pattern is not a
`TypedPattern`. As such, tweak
`RequirementFailure::getDeclRef` to handle this
case.
We probably ought to reconsider how we handle
contextual types here, using a ContextualType
locator when there is no contextual type recorded
seems a bit weird, though in most cases we want
to treat initializations the same regardless of
if a TypedPattern was used. For now I'm leaving
that as future work.
rdar://111009224