MDEV-35687 Various UBSAN function-type-mismatch debug_sync and myisam

storage/maria/ma_open.c:352:7: runtime error: call to function debug_sync(THD*, char const*, unsigned long)
through pointer to incorrect function type 'void (*)(void *, const char *, unsigned long)'

The THD argument is a void *. Because of the way myisam is .c files the
function prototype is mismatched.

As Marko pointed out the MYSQL_THD is declared as void * in C.

Thanks Jimmy Hú for noting that struct THD is the equalivalant in C to
the class THD. The C NULL was also different to the C++ nullptr.

Corrected the definations of MYSQL_THD and DEBUG_SYNC_C to be C and C++
compatible.

include/mysql/plugin.h

@@ -44,7 +44,8 @@ class THD;
 class Item;
 #define MYSQL_THD THD*
 #else
-#define MYSQL_THD void*
+struct THD;
+typedef struct THD* MYSQL_THD;
 #endif
 
 typedef char my_bool;