[Serialization] Mark decls that can never be cross-referenced (#17223)

This allows us to filter them out in cases that would otherwise be
ambiguous. The particular prompting situation looks a lot like the
test case: a protocol, plus a forward-declared class with the same
name. (Normally we ignore forward-declared classes, but SourceKit's
module interface generation feature makes dummy ClassDecls for them
instead.)

https://bugs.swift.org/browse/SR-4851

lib/Serialization/Serialization.cpp

@@ -603,6 +603,10 @@ DeclID Serializer::addDeclRef(const Decl *D, bool forceSerialization,
           isa<PrecedenceGroupDecl>(D)) &&
          "cannot cross-reference this decl");
 
+  assert((!isDeclXRef(D) ||
+          !D->getAttrs().hasAttribute<ForbidSerializingReferenceAttr>()) &&
+         "cannot cross-reference this decl");
+
   assert((allowTypeAliasXRef || !isa<TypeAliasDecl>(D) ||
           D->getModuleContext() == M) &&
          "cannot cross-reference typealiases directly (use the NameAliasType)");