* Update Decimal.swift
Fix `Decimal.magnitude` so that `Decimal.nan.magnitude` is no longer incorrectly `0`.
Port changes from apple/swift-corelibs-foundation#1759, crucially correcting an error where `(0 as Decimal).doubleValue` returned `.nan`.
* [gardening] Zap indentation error
* Add test for `Decimal.nan.magnitude`
* Apply reviewer suggestion
Clean up `doubleValue`
The SDK directory is now confusing as the Windows target also has a SDK
overlay. In order to make this more uniform, move the SDK directory to
Darwin which covers the fact that this covers the XNU family of OSes.
The Windows directory contains the SDK overlay for the Windows target.