Silence bogus GCC 7 warnings -Wimplicit-fallthrough

Do not silence uncertain cases, or fix any bugs.

The only functional change should be that ha_federated::extra()
is not calling DBUG_PRINT to report an unhandled case for
HA_EXTRA_PREPARE_FOR_DROP.

sql/sql_lex.cc

@@ -1,5 +1,5 @@
 /* Copyright (c) 2000, 2014, Oracle and/or its affiliates.
-   Copyright (c) 2009, 2016, MariaDB
+   Copyright (c) 2009, 2017, MariaDB
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -1395,12 +1395,14 @@ static int lex_one_token(YYSTYPE *yylval, THD *thd)
 	state= MY_LEX_HEX_NUMBER;
 	break;
       }
+      /* fall through */
     case MY_LEX_IDENT_OR_BIN:
       if (lip->yyPeek() == '\'')
       {                                 // Found b'bin-number'
         state= MY_LEX_BIN_NUMBER;
         break;
       }
+      /* fall through */
     case MY_LEX_IDENT:
       const char *start;
 #if defined(USE_MB) && defined(USE_MB_IDENT)
@@ -1745,6 +1747,7 @@ static int lex_one_token(YYSTYPE *yylval, THD *thd)
 	break;
       }
       /* " used for strings */
+      /* fall through */
     case MY_LEX_STRING:			// Incomplete text string
     {
       uint sep;