swift234

Don't bind generic type parameters to non-materializable types.

Add a new option, TVO_MustBeMaterializable, to
TypeVariableType::Implementation, and set it for type variables
resulting from opening a generic type. This solution isn't complete (we
don't yet copy the non-materializable bit on unification of type
variables, and it's possible to bind a must-be-materializable type
variable to a type with type variables that later get bound to
non-materializable types) but it addresses all reported crashes for this
issue.

<rdar://problem/20807269> Crash in non-materializable type

Swift SVN r28792