AST: Remove TypeDecl::getDeclaredType()

A pointless use of polymorphism -- the result values are not
interchangeable in any practical sense:

- For GenericTypeParamDecls, this returned getDeclaredInterfaceType(),
  which is an interface type.

- For AssociatedTypeDecls, this returned the sugared AssociatedTypeType,
  which desugars to an archetype.

- For TypeAliasDecls, this returned TypeAliasDecl::getAliasType(),
  which desugars to a type containing archetypes.

- For NominalTypeDecls, this returned NominalTypeDecl::getDeclaredType(),
  which is the unbound generic type, a special case used for inferring
  generic arguments when they're not written in source.

lib/Serialization/Serialization.cpp

@@ -4061,7 +4061,7 @@ void Serializer::writeAST(ModuleOrSourceFile DC) {
       hasLocalTypes = true;
 
       Mangle::Mangler DebugMangler(false);
-      DebugMangler.mangleType(TD->getDeclaredType(), 0);
+      DebugMangler.mangleType(TD->getDeclaredInterfaceType(), 0);
       auto MangledName = DebugMangler.finalize();
       assert(!MangledName.empty() && "Mangled type came back empty!");
       localTypeGenerator.insert(MangledName, {