Add annotations in a few places to silence some warnings

Author: Sainan

src/lcorolib.cpp

@@ -226,7 +226,7 @@ static int luaB_close (lua_State *L) {
       if (lua_tothread(L, -1) == co)
         return luaL_error(L, "cannot close main thread");
       lua_closethread(co, L);  /* close itself */
-      /* previous call does not return *//* FALLTHROUGH */
+      /* previous call does not return */SOUP_UNREACHABLE;
     default:
       lua_assert(0);
       return 0;

src/lparser.cpp

@@ -974,7 +974,7 @@ static void check_readonly (LexState *ls, expdesc *e) {
     case VVARGIND: {
       needvatab(fs->f);  /* function will need a vararg table */
       e->k = VINDEXED;
-    }  /* FALLTHROUGH */
+    } [[fallthrough]];
     case VINDEXUP: case VINDEXSTR: case VINDEXED: {  /* global variable */
       if (e->u.ind.ro)  /* read-only? */
         varname = tsvalue(&fs->f->k[e->u.ind.keystr]);
@@ -6272,7 +6272,7 @@ static void statement (LexState *ls, tdn_t *nprop, TypeHint *prop) {
       }  /* else... */
     }
 #endif
-    /* FALLTHROUGH */
+    [[fallthrough]];
     default: {  /* stat -> func | assignment */
       exprstat(ls);
       break;

src/luac.cpp

@@ -685,6 +685,7 @@ static void PrintCode(const Proto* f)
    case OP_EXTRAARG:
 	printf("%d",ax);
 	break;
+   case NUM_OPCODES: [[unreachable]];
 #if 0
    default:
 	printf("%d %d %d",a,b,c);