swift234

[serialization] Handle PolymorphicFunctionTypes more carefully.

PolymorphicFunctionTypes are built on GenericParamLists, which are
owned by decls that introduce generic parameters. This made deserializing
them a pain, because they /cannot/ be recreated in isolation -- the decl
provides some context. Rather than try to come up with a very generic way
to serialize these, this patch just records the GenericParamList of every
decl, allowing the type to be serialized with a reference to the decl
instead of to the param list. Deserialization can then just pluck the list
back out of the decl.

According to Doug and Joe, PolymorphicFunctionType's due for a refresh
anyway, so this kind of hackery / kludgery is acceptable.

With this change, we can now handle generic structs and generic functions
inside generic structs.

Swift SVN r6032