[OpaqueValues] AnyHashable src gets new context.
When storing an instance of some type that conforms to Hashable into an
lvalue of type AnyHashable, the source rvalue needs to be emitted within
a new SGFContext. Otherwise, the LocalVarInitialization for the var of
type AnyHashable would be used and an attempt would be made to store the
instance of the conforming type into the projected box.
