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/Sema/ConstraintSystem.cpp

@@ -1084,8 +1084,11 @@ ConstraintSystem::getTypeOfMemberReference(
     }
 
     // FIXME: Totally bogus fallthrough.
-    Type memberTy = isTypeReference? assocType->getDeclaredType()
-                                   : assocType->getType();
+    Type memberTy = isTypeReference
+        ? assocType->getDeclaredInterfaceType()
+        : assocType->getInterfaceType();
+    memberTy = ArchetypeBuilder::mapTypeIntoContext(
+        assocType->getProtocol(), memberTy);
     auto openedType = FunctionType::get(baseObjTy, memberTy);
     return { openedType, memberTy };
   }