Convert CNG into C++ and remove it from AudioDecoder

webrtc/modules/audio_coding/codecs/cng/cng_helpfuns.c

-/*
- *  Copyright (c) 2011 The WebRTC project authors. All Rights Reserved.
- *
- *  Use of this source code is governed by a BSD-style license
- *  that can be found in the LICENSE file in the root of the source
- *  tree. An additional intellectual property rights grant can be found
- *  in the file PATENTS.  All contributing project authors may
- *  be found in the AUTHORS file in the root of the source tree.
- */
-
-#include "cng_helpfuns.h"
-
-#include "signal_processing_library.h"
-#include "webrtc/typedefs.h"
-#include "webrtc_cng.h"
-
-/* Values in |k| are Q15, and |a| Q12. */
-void WebRtcCng_K2a16(int16_t* k, int useOrder, int16_t* a) {
-  int16_t any[WEBRTC_SPL_MAX_LPC_ORDER + 1];
-  int16_t *aptr, *aptr2, *anyptr;
-  const int16_t *kptr;
-  int m, i;
-
-  kptr = k;
-  *a = 4096;  /* i.e., (Word16_MAX >> 3) + 1 */
-  *any = *a;
-  a[1] = (*k + 4) >> 3;
-  for (m = 1; m < useOrder; m++) {
-    kptr++;
-    aptr = a;
-    aptr++;
-    aptr2 = &a[m];
-    anyptr = any;
-    anyptr++;
-
-    any[m + 1] = (*kptr + 4) >> 3;
-    for (i = 0; i < m; i++) {
-      *anyptr++ = (*aptr++) +
-          (int16_t)((((int32_t)(*aptr2--) * (int32_t) * kptr) + 16384) >> 15);
-    }
-
-    aptr = a;
-    anyptr = any;
-    for (i = 0; i < (m + 2); i++) {
-      *aptr++ = *anyptr++;
-    }
-  }
-}