Calendar: remove unnecessary return assignments (#1511)

Cherry-pick of
swiftlang/swift@68e7653

Author: ianpartridge

Foundation/Calendar.swift

@@ -890,25 +890,25 @@ public struct Calendar : Hashable, Equatable, ReferenceConvertible, _MutableBoxi
 
         switch matchingPolicy {
         case .nextTime:
-            let _ = result.insert(.matchNextTime)
+            result.insert(.matchNextTime)
         case .nextTimePreservingSmallerComponents:
-            let _ = result.insert(.matchNextTimePreservingSmallerUnits)
+            result.insert(.matchNextTimePreservingSmallerUnits)
         case .previousTimePreservingSmallerComponents:
-            let _ = result.insert(.matchPreviousTimePreservingSmallerUnits)
+            result.insert(.matchPreviousTimePreservingSmallerUnits)
         case .strict:
-            let _ = result.insert(.matchStrictly)
+            result.insert(.matchStrictly)
         }
 
         switch repeatedTimePolicy {
         case .first:
-            let _ = result.insert(.matchFirst)
+            result.insert(.matchFirst)
         case .last:
-            let _ = result.insert(.matchLast)
+            result.insert(.matchLast)
         }
 
         switch direction {
         case .backward:
-            let _ = result.insert(.searchBackwards)
+            result.insert(.searchBackwards)
         case .forward:
             break
         }
@@ -937,7 +937,7 @@ public struct Calendar : Hashable, Equatable, ReferenceConvertible, _MutableBoxi
 
         var result = NSCalendar.Unit()
         for u in units {
-            let _ = result.insert(unitMap[u]!)
+            result.insert(unitMap[u]!)
         }
         return result
     }