Factor generic parameters and associated types into their own decl nodes.

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

lib/Serialization/ModuleFile.cpp

@@ -72,7 +72,6 @@ validateControlBlock(llvm::BitstreamCursor &cursor,
   return result;
 }
 
-
 /// Used to deserialize entries in the on-disk decl hash table.
 class ModuleFile::DeclTableInfo {
 public: