Auto merge of #6685 - magurotuna:filter_map_identity, r=phansch

Add new lint `filter_map_identity`

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This commit adds a new lint named filter_map_identity.
This lint is the same as `flat_map_identity` except that it checks for the usage of `filter_map`.

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Closes #6643

changelog: Added a new lint: `filter_map_identity`

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