Unused return value checker

This is an issue to discuss **Unused return value checker** proposal. The text of the proposal is located [here](https://github.com/Kotlin/KEEP/blob/underscore-for-unused-local/proposals/unused-return-value-checker.md). This proposal depends on the other sub-proposal, **Underscore for unused local variables**, located [here](https://github.com/Kotlin/KEEP/blob/underscore-for-unused-local/proposals/underscores-for-local-variables.md). Since both proposals are connected, we'll discuss them together. The goal of the proposals is to enhance the functionality of the existing 'unused expression' diagnostic in the Kotlin compiler. New diagnostic should be able to report complex expressions whose return values are meaningful (e.g., non-Unit) but are not used.