Adding [[fallthrough]] annotation to case statements in faiss/utils/h…

…amming_distance/neon-inl.h

Summary:
Error: https://www.internalfb.com/sandcastle/workflow/1661828262501424314
Due to a recent change (D52393687) to increase compiler warnings arcoss faiss/PACKAGE aggregator build is failing. The annotation [[fallthrough]]; needs to be used in every [nested]case statement where fallthrough can occur.

WHAT I DIDNT DO
I did NOT EXPORTED TO OSS FAISS
mdouze please do this if it is required. I do not know your policy. Prior to landing i saw the message but was unsure. As well as not having github account.

Reviewed By: r-barnes

Differential Revision: D52420730

fbshipit-source-id: 92ec8382226ea35bdacb668b3c329bd8b41a67fc

faiss/utils/hamming_distance/neon-inl.h

@@ -341,21 +341,27 @@ struct HammingComputerDefault {
  case 7:
  accu += popcount64(a64[i] ^ b64[i]);
  i++;
+ [[fallthrough]];
  case 6:
  accu += popcount64(a64[i] ^ b64[i]);
  i++;
+ [[fallthrough]];
  case 5:
  accu += popcount64(a64[i] ^ b64[i]);
  i++;
+ [[fallthrough]];
  case 4:
  accu += popcount64(a64[i] ^ b64[i]);
  i++;
+ [[fallthrough]];
  case 3:
  accu += popcount64(a64[i] ^ b64[i]);
  i++;
+ [[fallthrough]];
  case 2:
  accu += popcount64(a64[i] ^ b64[i]);
  i++;
+ [[fallthrough]];
  case 1:
  accu += popcount64(a64[i] ^ b64[i]);
  i++;
@@ -367,18 +373,25 @@ struct HammingComputerDefault {
  switch (remainder8) {
  case 7:
  accu += hamdis_tab_ham_bytes[a[6] ^ b[6]];
+ [[fallthrough]];
  case 6:
  accu += hamdis_tab_ham_bytes[a[5] ^ b[5]];
+ [[fallthrough]];
  case 5:
  accu += hamdis_tab_ham_bytes[a[4] ^ b[4]];
+ [[fallthrough]];
  case 4:
  accu += hamdis_tab_ham_bytes[a[3] ^ b[3]];
+ [[fallthrough]];
  case 3:
  accu += hamdis_tab_ham_bytes[a[2] ^ b[2]];
+ [[fallthrough]];
  case 2:
  accu += hamdis_tab_ham_bytes[a[1] ^ b[1]];
+ [[fallthrough]];
  case 1:
  accu += hamdis_tab_ham_bytes[a[0] ^ b[0]];
+ [[fallthrough]];
  default:
  break;
  }