Replace fallthrough comments with __fallthrough__

Defined in config.h

src/common.cpp

@@ -1071,7 +1071,7 @@ static wcstring escape_string_pcre2(const wcstring &in) {
             case L'-':
             case L']':
                 out.push_back('\\');
-                /* FALLTHROUGH */
+                __fallthrough__
             default:
                 out.push_back(c);
         }

src/expand.cpp

@@ -947,7 +947,7 @@ expand_result_t expander_t::stage_cmdsubst(wcstring input, completion_receiver_t
                 return expand_result_t::ok;
             case 1:
                 append_cmdsub_error(errors, start, end, L"command substitutions not allowed here");
-                /* intentionally falls through */
+                __fallthrough__
             case -1:
             default:
                 return expand_result_t::make_error(STATUS_EXPAND_ERROR);

src/parse_util.cpp

@@ -377,8 +377,8 @@ static void job_or_process_extent(bool process, const wchar_t *buff, size_t curs
                 if (!process) {
                     break;
                 }
+                __fallthrough__
             }
-            /* FALLTHROUGH */
             case token_type_t::end:
             case token_type_t::background:
             case token_type_t::andand: