Add @discardableResult attribute to stablePartition

[adincebic]

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Since stablePartition is used frequently in contexts where it's return result isn't needed, I added @discardableResult attribute to both public methods for stablePartition to avoid Xcode's warning about unused result of call to stablePartition.

Checklist

• [ ] I've added at least one test that validates that my change is working, if appropriate
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [ ] I've updated the documentation if necessary

[adincebic]

@swift-ci Please smoke test

[glessard]

@swift-ci please test

[glessard]

Why add @discardableResult on stablePartition but not partition?

[natecook1000]

Thanks for your contribution, @adincebic! I don't think there are frequent cases where you want to partition a collection but don't care about where the partition is — partitioning is generally used as part of a larger algorithm, and further processing depends on the position of the partition. Feel free to start a thread on the Swift forums if you'd like to discuss further!