[clang] [DirectoryWatcher] Remove leading \\?\ from GetFinalPathNameByHandleW

[mstorsjo]

Summary

This is needed for the paths to work when using forward slashes;
this fixes the DirectoryWatcherTests unit tests.

Also allocate missing space for the null terminator, which seems to
have been missing all along (writing the terminator out of bounds).

[aaron.ballman]

LGTM, though I'd appreciate if you'd fix the clang-format issues when landing.

clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp

 DirectoryWatcherWindows::DirectoryWatcherWindows(
     HANDLE DirectoryHandle, bool WaitForInitialSync,
     DirectoryWatcherCallback Receiver)
     : Callback(Receiver), Terminate(INVALID_HANDLE_VALUE) {
   // Pre-compute the real location as we will be handing over the directory
   // handle to the watcher and performing synchronous operations.
   {
     DWORD Size = GetFinalPathNameByHandleW(DirectoryHandle, NULL, 0, 0);
-    std::unique_ptr<WCHAR[]> Buffer{new WCHAR[Size]};
+    std::unique_ptr<WCHAR[]> Buffer{new WCHAR[Size+1]};
     Size = GetFinalPathNameByHandleW(DirectoryHandle, Buffer.get(), Size, 0);
     Buffer[Size] = L'\0';
-    llvm::sys::windows::UTF16ToUTF8(Buffer.get(), Size, Path);
+    WCHAR *Data = Buffer.get();
+    if (Size >= 4 && ::memcmp(Data, L"\\\\?\\", 8) == 0) {
+      Data += 4;
+      Size -= 4;
+    }
+    llvm::sys::windows::UTF16ToUTF8(Data, Size, Path);
   }
 
   size_t EntrySize = sizeof(FILE_NOTIFY_INFORMATION) + MAX_PATH * sizeof(WCHAR);
   Notifications.resize((4 * EntrySize) / sizeof(DWORD));
 
   memset(&Overlapped, 0, sizeof(Overlapped));
   Overlapped.hEvent =
       CreateEventW(NULL, /*bManualReset=*/FALSE, /*bInitialState=*/FALSE, NULL);