[SR-6785] Fix Decimal conformance to Strideable

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Consider the following discrepancy:

import Foundation

let x: Int = 42
x.distance(to: 43) // 1
let y: Decimal = 42
y.distance(to: 43) // -1 (oops!)

The implementation of distance(to:) is incorrect for Decimal. This PR corrects the implementation (for both the overlay and swift-corelibs) and adds corresponding tests.

Resolves SR-6785.

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@swift-ci test

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@swift-ci test

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@swift-ci please test Linux

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@swift-ci test