[stdlib] NFC: Fix var to let warning

Author: davezarzycki

stdlib/public/core/BridgeObjectiveC.swift

@@ -788,7 +788,7 @@ public func _bridgeAnythingToObjectiveC<T>(_ x: T) -> AnyObject {
   var done = false
   var result: AnyObject!
 
-  var source: Any = x
+  let source: Any = x
 
   if let dynamicSource = _extractDynamicValue(x) {
     result = dynamicSource as AnyObject