Foundation: support long file paths for FileHandle on Windows

When constructing a `FileHandle` using a path, ensure that we use the NT
file path representation on Windows.  This allows us to access files
that may have greater than `MAX_PATH` length.

Sources/Foundation/FileHandle.swift

@@ -432,12 +432,12 @@ open class FileHandle : NSObject {
     }
 
     internal convenience init?(path: String, flags: Int32, createMode: Int) {
-      guard let fd: Int32 = try? FileManager.default._fileSystemRepresentation(withPath: path, {
-        _CFOpenFileWithMode($0, flags, mode_t(createMode))
-      }), fd > 0 else { return nil }
+        guard let fd: CInt = try? withNTPathRepresentation(of: path, {
+            _CFOpenFileWithMode($0, flags, mode_t(createMode))
+        }), fd > 0 else { return nil }
 
-      self.init(fileDescriptor: fd, closeOnDealloc: true)
-      if self._handle == INVALID_HANDLE_VALUE { return nil }
+        self.init(fileDescriptor: fd, closeOnDealloc: true)
+        if self._handle == INVALID_HANDLE_VALUE { return nil }
     }
 #else
     public init(fileDescriptor fd: Int32, closeOnDealloc closeopt: Bool) {

Sources/Foundation/FileManager+Win32.swift

@@ -30,7 +30,7 @@ extension URL {
 }
 
 
-private func withNTPathRepresentation<Result>(of path: String, _ body: (UnsafePointer<WCHAR>) throws -> Result) throws -> Result {
+internal func withNTPathRepresentation<Result>(of path: String, _ body: (UnsafePointer<WCHAR>) throws -> Result) throws -> Result {
     guard !path.isEmpty else {
         throw CocoaError.error(.fileReadInvalidFileName, userInfo: [NSFilePathErrorKey:path])
     }
@@ -72,6 +72,9 @@ private func withNTPathRepresentation<Result>(of path: String, _ body: (UnsafePo
             _ = GetFullPathNameW(pwszPath, DWORD($0.count), $0.baseAddress, nil)
             return String(decodingCString: $0.baseAddress!, as: UTF16.self)
         }
+        guard !path.hasPrefix(#"\\"#) else {
+            return try path.withCString(encodedAs: UTF16.self, body)
+        }
         return try #"\\?\\#(path)"#.withCString(encodedAs: UTF16.self, body)
     }
 }