src: use explicit C++17 fallthrough

This passes the strictest -Wimplicit-fallthrough setting.

PR-URL: #46251
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/crypto/crypto_clienthello.cc

@@ -29,7 +29,7 @@ void ClientHelloParser::Parse(const uint8_t* data, size_t avail) {
     case kWaiting:
       if (!ParseRecordHeader(data, avail))
         break;
-      // Fall through
+      [[fallthrough]];
     case kTLSHeader:
       ParseHeader(data, avail);
       break;

src/inspector_io.cc

@@ -74,7 +74,7 @@ class RequestToServer {
     switch (action_) {
       case TransportAction::kKill:
         server->TerminateConnections();
-        // Fallthrough
+        [[fallthrough]];
       case TransportAction::kStop:
         server->Stop();
         break;

src/node_i18n.cc

@@ -794,12 +794,12 @@ static int GetColumnWidth(UChar32 codepoint,
         return 2;
       }
       // If ambiguous_as_full_width is false:
-      // Fall through
+      [[fallthrough]];
     case U_EA_NEUTRAL:
       if (u_hasBinaryProperty(codepoint, UCHAR_EMOJI_PRESENTATION)) {
         return 2;
       }
-      // Fall through
+      [[fallthrough]];
     case U_EA_HALFWIDTH:
     case U_EA_NARROW:
     default:

src/node_zlib.cc

@@ -797,7 +797,7 @@ void ZlibContext::DoThreadPoolWork() {
             break;
           }
 
-          // fallthrough
+          [[fallthrough]];
         case 1:
           if (next_expected_header_byte == nullptr) {
             break;
@@ -817,7 +817,7 @@ void ZlibContext::DoThreadPoolWork() {
           CHECK(0 && "invalid number of gzip magic number bytes read");
       }
 
-      // fallthrough
+      [[fallthrough]];
     case INFLATE:
     case GUNZIP:
     case INFLATERAW: