quickfix suggestion for solidity 6480 derived contract must override function

[kanej]

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In https://github.com/nomiclabs/hardhat-vscode/pull/38 we provided functionality to implement missing functions from an interface. solidity 6480 is related to solidty 3656 and resolving solidity 3656 resolves solidity 6480, but sometimes solidity 3656 appears on its own. We should allow the reuse the code for solving solidity 6480 to allow a quickfix when it occurs by itself.

We should also consider whether to hide solidity 6480 in the presence of solidty 3656 to reduce clutter.