Upgrade path for Config translation synchronization failure

Adding all the related issues.

Discussed with @xjm, @webchick, @effulgentsia and @Cottser. We agreed to keep this critical because:

1. We haven't confirmed that we're still not facing the issue with config sync discovered in #2750167: Config translation synchronization fails has been fixed on existing sites
2. Existing sites which have config in this situation will have expected config change when they do anything that re-saves this config

Here's the whack it with a hammer upgrade path approach.

The beta allowed changes include critical issues, so the beta target tag is not applicable.

I agree with alex, its not really worth to try to optimize the update here. In case we miss any case, it will hurt much more, than a little bit more time to run the update.

1. +++ b/core/modules/system/src/Tests/Update/ConfigOverridesUpdateTest.php
   @@ -0,0 +1,55 @@
   +    $this->assertFalse($config_override->get($key_to_be_removed), 'The spanish override for the missing field has been removed.');
   

   Can we use something like assertNull?

2. +++ b/core/modules/system/src/Tests/Update/ConfigOverridesUpdateTest.php
   @@ -0,0 +1,55 @@
   +    $this->assertTrue($config_override->isNew() && empty($config_override->get()), 'After updating the system.cron spanish override does not exist.');
   

   Its weird to not have splitted up this into two assertions. It makes debugging harder than it would have to be

Re #9.2 I did that because isNew() should be enough and doing it on separate lines looks weird to me. But so be it... here's the change.

Thank you alex!

Let's hope no one is doing anything made on config save... mind you if they are config import would probably be broken too.

Wow, thanks!