This is a baby step toward eliminating GenericParamList from
PolymorphicFunctionType, fixing up the easy callers. No functionality
change.
Swift SVN r7370
Previously, TypeAliasDecl was used for typealiases, generic
parameters, and assocaited types, which is hideous and the source of
much confusion. Factor the latter two out into their own decl nodes,
with a common abstract base for "type parameters", and push these
nodes throughout the frontend.
No real functionality change, but this is a step toward uniquing
polymorphic types, among other things.
Swift SVN r7345