Fix two Decimal members with respect to NaN

[xwu]

This PR corresponds to swiftlang/swift#25698.

It fixes Decimal.magnitude so that Decimal.nan.magnitude is no longer incorrectly 0.

The corresponding PR in the apple/swift repository also incorporates changes from #1759; here, minor edits (NFC), including one suggested on review, are back-propagated so as to help the two versions of Foundation.Decimal remain in sync.

Resolves SR-10938.

[xwu]

@swift-ci Please test

[xwu]

@spevans There's at least one other fix that isn't synchronized between here and the overlay. One thing I'd like to do (or rather, have already done but would like to contribute in a PR) is to reorganize Decimal.swift and bring the two implementations back into alignment where it makes sense to do so. Even the organization of two files has fallen out of sync, making it increasingly difficult to ensure parity. Does that sound sensible to you?

[spevans]

This looks fine but might be worth adding a few tests for .magnitude of various numbers, eg the Decimal constants Decimal.leastFiniteMagnitude, Decimal.greatestFiniteMagnitude etc and maybe Decimal(1), Decimal(-1)

[spevans]

Yes, definitely a good idea to resync the two versions. I think the scl-f version may be slightly ahead in bug fixes but there should be enough tests in it to cover any slight regressions. Probably worth doing a couple of PRs to resync them.

[spevans]

Could you just squash these down into one commit and then it can be merged, thanks

[xwu]

@swift-ci Please test Linux