Fix crash when proc return type is undeclared parapoly variable

Disallow the declaration of new parapoly variables in return types, when
the procedure's parapoly scope is itself. This happens if e.g.:
`foo :: proc() -> $T`.

Closes #3949, #4294, #4563

src/checker.hpp

@@ -521,6 +521,7 @@ struct CheckerContext {
 	bool       in_enum_type;
 	bool       collect_delayed_decls;
 	bool       allow_polymorphic_types;
+	bool       disallow_polymorphic_return_types; // NOTE(zen3ger): no poly type decl in return types
 	bool       no_polymorphic_errors;
 	bool       hide_polymorphic_errors;
 	bool       in_polymorphic_specialization;