swift234

[serialization] Add support for generic functions.

This currently does a little dance to handle PolymorphicFunctionTypes.
These are currently implemented by referencing the GenericParamList of
an actual polymorphic function, so they can't be deserialized without
knowing which function they are attached to. To solve this,
PolymorphicFunctionTypes are serialized as regular FunctionTypes. Then,
when a generic function decl is being deserialized, it will rebuild
a PolymorphicFunctionType from the serialized type and its own generic
parameter list.

Requirements on the generic types are coming next.

Swift SVN r5983