Auto merge of #4008 - g-bartoszek:boxed-fnmut, r=phansch

Do not trigger redundant_closure for non-function types

fixes #3898

Added a check for the entity being called in the closure body to be a FnDef. This way lint does not trigger for ADTs (Box) but I'm not sure if it's correct and not too restrictive.

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changelog: Fix false positive in `redundant_closure` pertaining to non-function types

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