[Foundation] Fix two Decimal members with respect to NaN (#25698)

* Update Decimal.swift

Fix `Decimal.magnitude` so that `Decimal.nan.magnitude` is no longer incorrectly `0`.
Port changes from swiftlang/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`

Author: xwu

Diff for: stdlib/public/Darwin/Foundation/Decimal.swift

@@ -150,21 +150,32 @@ public func pow(_ x: Decimal, _ y: Int) -> Decimal {
 }
 
 extension Decimal : Hashable, Comparable {
-    internal var doubleValue : Double {
+    private subscript(index: UInt32) -> UInt16 {
+        get {
+            switch index {
+            case 0: return _mantissa.0
+            case 1: return _mantissa.1
+            case 2: return _mantissa.2
+            case 3: return _mantissa.3
+            case 4: return _mantissa.4
+            case 5: return _mantissa.5
+            case 6: return _mantissa.6
+            case 7: return _mantissa.7
+            default: fatalError("Invalid index \(index) for _mantissa")
+            }
+        }
+    }
+    
+    internal var doubleValue: Double {
+        if _length == 0 {
+            return _isNegative == 1 ? Double.nan : 0
+        }
+
         var d = 0.0
-        if _length == 0 && _isNegative == 0 {
-            return Double.nan
+        for idx in (0..<min(_length, 8)).reversed() {
+            d = d * 65536 + Double(self[idx])
         }
 
-        d = d * 65536 + Double(_mantissa.7)
-        d = d * 65536 + Double(_mantissa.6)
-        d = d * 65536 + Double(_mantissa.5)
-        d = d * 65536 + Double(_mantissa.4)
-        d = d * 65536 + Double(_mantissa.3)
-        d = d * 65536 + Double(_mantissa.2)
-        d = d * 65536 + Double(_mantissa.1)
-        d = d * 65536 + Double(_mantissa.0)
-        
         if _exponent < 0 {
             for _ in _exponent..<0 {
                 d /= 10.0
@@ -212,6 +223,7 @@ extension Decimal : ExpressibleByIntegerLiteral {
 
 extension Decimal : SignedNumeric {
   public var magnitude: Decimal {
+      guard _length != 0 else { return self }
       return Decimal(
           _exponent: self._exponent, _length: self._length,
           _isNegative: 0, _isCompact: self._isCompact,

Diff for: test/stdlib/TestDecimal.swift

@@ -303,6 +303,7 @@ class TestDecimal : TestDecimalSuper {
         expectEqual(Decimal(68040), Decimal(386).advanced(by: Decimal(67654)))
         expectEqual(Decimal(1.234), abs(Decimal(1.234)))
         expectEqual(Decimal(1.234), abs(Decimal(-1.234)))
+        expectTrue(Decimal.nan.magnitude.isNaN)
         var a = Decimal(1234)
         var r = a
         expectEqual(.noError, NSDecimalMultiplyByPowerOf10(&r, &a, 1, .plain))