Mark all _unconditionallyBridgeFromObjectiveC functions as @_effects(readonly)

This enables removal of those bridging calls for dead parameters. Read-only lets the optimizer remove such a call if the result is not used.
Note that "readonly" means: no observable write operations. It's okay to allocate and initialize new objects.

rdar://problem/44944094

Author: eeckstein

Darwin/Foundation-swiftoverlay/AffineTransform.swift

@@ -317,6 +317,7 @@ extension AffineTransform : _ObjectiveCBridgeable {
         return true // Can't fail
     }
 
+    @_effects(readonly)
     public static func _unconditionallyBridgeFromObjectiveC(_ x: NSAffineTransform?) -> AffineTransform {
         guard let src = x else { return AffineTransform.identity }
         return AffineTransform(reference: src)

Darwin/Foundation-swiftoverlay/Calendar.swift

@@ -1113,6 +1113,7 @@ extension Calendar : _ObjectiveCBridgeable {
         return true
     }
 
+    @_effects(readonly)
     public static func _unconditionallyBridgeFromObjectiveC(_ source: NSCalendar?) -> Calendar {
         var result: Calendar?
         _forceBridgeFromObjectiveC(source!, result: &result)

Darwin/Foundation-swiftoverlay/CharacterSet.swift

@@ -782,6 +782,7 @@ extension CharacterSet : _ObjectiveCBridgeable {
         return true
     }
 
+    @_effects(readonly)
     public static func _unconditionallyBridgeFromObjectiveC(_ source: NSCharacterSet?) -> CharacterSet {
         guard let src = source else { return CharacterSet() }
         return CharacterSet(_bridged: src)

Darwin/Foundation-swiftoverlay/Data.swift

@@ -1902,7 +1902,8 @@ extension Data : _ObjectiveCBridgeable {
         result = Data(referencing: input)
         return true
     }
-    
+
+    @_effects(readonly)
     public static func _unconditionallyBridgeFromObjectiveC(_ source: NSData?) -> Data {
         guard let src = source else { return Data() }
         return Data(referencing: src)

Darwin/Foundation-swiftoverlay/Date.swift

@@ -257,6 +257,7 @@ extension Date : _ObjectiveCBridgeable {
         return true
     }
 
+    @_effects(readonly)
     public static func _unconditionallyBridgeFromObjectiveC(_ source: NSDate?) -> Date {
         var result: Date?
         _forceBridgeFromObjectiveC(source!, result: &result)

Darwin/Foundation-swiftoverlay/DateComponents.swift

@@ -338,6 +338,7 @@ extension DateComponents : _ObjectiveCBridgeable {
         return true
     }
 
+    @_effects(readonly)
     public static func _unconditionallyBridgeFromObjectiveC(_ source: NSDateComponents?) -> DateComponents {
         guard let src = source else { return DateComponents() }
         return DateComponents(reference: src)

Darwin/Foundation-swiftoverlay/DateInterval.swift

@@ -216,6 +216,7 @@ extension DateInterval : _ObjectiveCBridgeable {
         return true
     }
 
+    @_effects(readonly)
     public static func _unconditionallyBridgeFromObjectiveC(_ source: NSDateInterval?) -> DateInterval {
         var result: DateInterval?
         _forceBridgeFromObjectiveC(source!, result: &result)

Darwin/Foundation-swiftoverlay/Decimal.swift

@@ -487,6 +487,7 @@ extension Decimal : _ObjectiveCBridgeable {
         return true
     }
 
+    @_effects(readonly)
     public static func _unconditionallyBridgeFromObjectiveC(_ source: NSDecimalNumber?) -> Decimal {
         guard let src = source else { return Decimal(_exponent: 0, _length: 0, _isNegative: 0, _isCompact: 0, _reserved: 0, _mantissa: (0, 0, 0, 0, 0, 0, 0, 0)) }
         return src.decimalValue

Darwin/Foundation-swiftoverlay/Foundation.swift

@@ -73,6 +73,7 @@ extension AnyHashable : _ObjectiveCBridgeable {
     return result != nil
   }
 
+  @_effects(readonly)
   public static func _unconditionallyBridgeFromObjectiveC(
     _ source: NSObject?
   ) -> AnyHashable {

Darwin/Foundation-swiftoverlay/IndexPath.swift

@@ -780,6 +780,7 @@ extension IndexPath : _ObjectiveCBridgeable {
         return true
     }
 
+    @_effects(readonly)
     public static func _unconditionallyBridgeFromObjectiveC(_ source: NSIndexPath?) -> IndexPath {
         guard let src = source else { return IndexPath() }
         return IndexPath(nsIndexPath: src)