Debugging help: print which MEM_ROOT the object is on

This is a debugging help function that was developed when reviewing fix for MDEV-35815.

Developers agreed that it's generally useful.

Targeting 10.6 as we are still debugging there, too, and this will have no impact on release builds.

Example of usage:
Where is "pos" located:

(gdb) p pos

$67 = (Item_direct_view_ref *) 0x7fff58b66a30

(gdb) p dbug_which_mem_root(thd, pos)

$68 = 0x55555703a912 "thd->stmt_arena->mem_root"

Ok, it is statement-lifetime mem_root.

What about pos->name? Where was it allocated?

(gdb) p pos->name.str

$69 = 0x7fff58017f10 "fld"

(gdb) p dbug_which_mem_root(thd, pos->name.str)

$70 = 0x55555703a904 "thd->mem_root"

On a transient mem_root. This was the bug in MDEV-35815 .