Bug#27660330 ADD SUPPORT FOR CLANG UBSAN bool system variables

Use 'bool' rather than 'char' for boolean system variables.

runtime error: call to function mysql_sys_var_bool(THD*, int)
through pointer to incorrect function type 'char *(*)(THD *, int)'
sql/sql_plugin.cc:2733: note: mysql_sys_var_bool(THD*, int) defined here

Change-Id: Ie712ed9be7134cf349f7ea236353849013a313e9

Author: Tor Didriksen

include/mysql/plugin.h

@@ -468,7 +468,7 @@ typedef void (*mysql_var_update_func)(MYSQL_THD thd, SYS_VAR *var,
       blk}
 
 #define MYSQL_THDVAR_BOOL(name, opt, comment, check, update, def)      \
-  DECLARE_MYSQL_THDVAR_BASIC(name, char) = {                           \
+  DECLARE_MYSQL_THDVAR_BASIC(name, bool) = {                           \
       PLUGIN_VAR_BOOL | PLUGIN_VAR_THDLOCAL | ((opt)&PLUGIN_VAR_MASK), \
       #name,                                                           \
       comment,                                                         \