flambda-backend: Turn upstream compatibility errors into a warning (#…

…2572)

* `Incompatible_with_upstream` warning

* Raise warnings instead of errors

* Fix tests

* Fix whitespace

* Edit warning messages

* Update test

* Basic example with disabling warnings

* Remove redundant warnings

* More tests

* More warnings tests

* Even more tests

* Eaxmples

* revert fix: done in another pr

* Demonstrate test case

* Update comments

* Demonstrate that we need to check expressions too

* Revert "Update comments"

This reverts commit c955bd8.

* Revert "Remove redundant warnings"

This reverts commit 3e87b64.

* Document unsoundness

* CR for duplicate warnings

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Co-authored-by: Diana Kalinichenko <dkalinichenko@janestreet.com>