swift234

Introduce a new representation of polymorphic function types.

Introduces a new kind of function type, GenericFunctionType, that
represents a polymorphic function type with all of its generic
parameters and requirements stored in a more readily canonicalizable
form. It is meant to eventually replace PolymorphicFunctionType, but
for now we build it up in parallel so we can switch over to it
pieacemeal.

Note: this representation is built and then thrown away. We'll start
recording it soon.


Swift SVN r8881