Fix sign of fractional units in Duration initializers from Double. (#66582)

* Fix sign of fractional units in Duration initializers from Double.

When separating a double duration into integral and fractional parts, we got the sign wrong. This fixes that bug.

* Fixup variable name.

Author: stephentyrone

stdlib/public/core/Duration.swift

@@ -122,7 +122,7 @@ extension Duration {
     // handle them slightly differently to ensure that integer values are
     // never rounded if `scale` is representable as Double.
     let integralPart = duration.rounded(.towardZero)
-    let fractionalPart = integralPart - duration
+    let fractionalPart = duration - integralPart
     self.init(_attoseconds:
       // This term may trap due to overflow, but it cannot round, so if the
       // input `seconds` is an exact integer, we get an exact integer result.

test/stdlib/Duration.swift

@@ -14,6 +14,8 @@ if #available(SwiftStdlib 5.7, *) {
       expectEqual(sec, Int64(integerValue))
       expectEqual(attosec, 0)
     }
+    let quarterSecond = Duration.seconds(0.25)
+    expectEqual(quarterSecond.components, (0, 250_000_000_000_000_000))
     // Value that overflows conversion from Double -> Int64, but should be
     // representable as a number of seconds:
     let huge: Double = 1.7e20