Fix divergent overwrite behavior of FileManager.copyItem(at:to:) on Linux and Windows (swiftlang#4821)

* Fix silent-overwrite behavior of FileManager.copyItem() on Linux

* Fix silent-overwrite behavior of FileManager.copyItem() on Windows

Author: gwynne

Sources/Foundation/FileManager+POSIX.swift

@@ -584,7 +584,7 @@ extension FileManager {
         }
         defer { close(srcfd) }
 
-        let dstfd = open(dstRep, O_WRONLY | O_CREAT | O_TRUNC, 0o666)
+        let dstfd = open(dstRep, O_WRONLY | O_CREAT | O_EXCL | O_TRUNC, 0o666)
         guard dstfd >= 0 else {
             throw _NSErrorWithErrno(errno, reading: false, path: dstPath,
                                     extraUserInfo: extraErrorInfo(srcPath: srcPath, dstPath: dstPath, userVariant: variant))

Sources/Foundation/FileManager+Win32.swift

@@ -550,7 +550,7 @@ extension FileManager {
     internal func _copyRegularFile(atPath srcPath: String, toPath dstPath: String, variant: String = "Copy") throws {
         try withNTPathRepresentation(of: srcPath) { wszSource in
             try withNTPathRepresentation(of: dstPath) { wszDestination in
-                if !CopyFileW(wszSource, wszDestination, false) {
+                if !CopyFileW(wszSource, wszDestination, true) {
                     throw _NSErrorWithWindowsError(GetLastError(), reading: true, paths: [srcPath, dstPath])
                 }
             }