Clear cache on reload

[GregoryTravis]

This uses #11577. It creates a reference to a value that is cleared on reload; this change detects this, and clears the cache.

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[JaroslavTulach]

Oh, oh, a new builtin!

I tried to keep the conceptual surface of the reload button API as small as possible and exposing release_references to general use goes against that.

I can see why it is useful in unit tests, but I hope we'll find another solution to the problem than a builtin available to everyone.