swift234

[serialization] Handle references to generic parameters.

Generic parameters are implemented using specially-tagged TypeAliasDecls.
Unlike normal ValueDecls, their names are not resilient, and so cross-
module references shouldn't refer to them by name. Instead, use an index
into the generic parameter list of their context.

Since generic parameters can appear within extensions, this new kind isn't
mutually exclusive with the just-introduced ExtensionValue. Change that to
be a separate flag that applies to both Values and GenericParameters.

Swift SVN r6304