[gardening] prefer '-> Void' over '-> ()'

This is same as swiftlang/swift#6521.

Author: ikesyo

Foundation/Data.swift

@@ -1390,7 +1390,7 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
             let shift = resultCount - currentCount
             let start = subrange.lowerBound
 
-            self.withUnsafeMutableBytes { (bytes : UnsafeMutablePointer<UInt8>) -> () in
+            self.withUnsafeMutableBytes { (bytes : UnsafeMutablePointer<UInt8>) -> Void in
                 if shift != 0 {
                     let destination = bytes + start + replacementCount
                     let source = bytes + start + subrangeCount

Foundation/NSData.swift

@@ -806,7 +806,7 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
         }()
 
         var currentLineCount = 0
-        let appendByteToResult : (UInt8) -> () = {
+        let appendByteToResult : (UInt8) -> Void = {
             result.append($0)
             currentLineCount += 1
             if let options = lineOptions, currentLineCount == options.lineLength {

Foundation/NSStringAPI.swift

@@ -422,7 +422,7 @@ extension String {
 
   /// Enumerates all the lines in a string.
   public func enumerateLines(
-    invoking body: @escaping (_ line: String, _ stop: inout Bool) -> ()
+    invoking body: @escaping (_ line: String, _ stop: inout Bool) -> Void
   ) {
     _ns.enumerateLines {
       (line: String, stop: UnsafeMutablePointer<ObjCBool>)
@@ -455,7 +455,7 @@ extension String {
     _ body: @escaping (
       _ substring: String?, _ substringRange: Range<Index>,
       _ enclosingRange: Range<Index>, inout Bool
-    ) -> ()
+    ) -> Void
   ) {
     _ns.enumerateSubstrings(in: _toNSRange(range), options: opts) {
       var stop_ = false
@@ -1587,7 +1587,7 @@ extension String {
     _ body: (
       _ substring: String?, _ substringRange: Range<Index>,
       _ enclosingRange: Range<Index>, inout Bool
-    ) -> ()
+    ) -> Void
   ) {
     fatalError("unavailable function can't be called")
   }

Foundation/NSURLSession/HTTPBodySource.swift

@@ -107,7 +107,7 @@ internal final class _HTTPBodyFileSource {
     fileprivate let fileURL: URL
     fileprivate let channel: DispatchIO 
     fileprivate let workQueue: DispatchQueue 
-    fileprivate let dataAvailableHandler: () -> ()
+    fileprivate let dataAvailableHandler: () -> Void
     fileprivate var hasActiveReadHandler = false
     fileprivate var availableChunk: _Chunk = .empty
     /// Create a new data source backed by a file.
@@ -121,7 +121,7 @@ internal final class _HTTPBodyFileSource {
     ///     no data may be available even if there's more data in the file.
     ///     if `getNextChunk(withLength:)` returns `.retryLater`, this handler
     ///     will be called once data becomes available.
-    init(fileURL: URL, workQueue: DispatchQueue, dataAvailableHandler: @escaping () -> ()) {
+    init(fileURL: URL, workQueue: DispatchQueue, dataAvailableHandler: @escaping () -> Void) {
         guard fileURL.isFileURL else { fatalError("The body data URL must be a file URL.") }
         self.fileURL = fileURL
         self.workQueue = workQueue

Foundation/NSURLSession/TaskRegistry.swift

@@ -45,7 +45,7 @@ extension URLSession {
 
         fileprivate var tasks: [Int: URLSessionTask] = [:]
         fileprivate var behaviours: [Int: _Behaviour] = [:]
-        fileprivate var tasksFinishedCallback: (() -> ())?
+        fileprivate var tasksFinishedCallback: (() -> Void)?
     }
 }
 
@@ -87,7 +87,7 @@ extension URLSession._TaskRegistry {
         }
     }
 
-    func notify(on tasksCompetion: @escaping () -> ()) {
+    func notify(on tasksCompetion: @escaping () -> Void) {
         tasksFinishedCallback = tasksCompetion
     }