VC++ has an intrinsic __is_convertible_to. I've started
implementing it, but here's the problem: The existing
type traits are implemented in ExprCXX.cpp, and are
passed an ASTContext only. However, the only way that
I see to implement this trait (using InitializationSequence)
requires a Sema which is not available in the places where
EvaluateTrait is called from. Any ideas?