User: support setting custom fields + array autocreation in non-existent field

User: support setting custom fields + array autocreation in non-existent field

I889924037 added a __set method which did not actually handle fields being set.
For better or worse, setting custom fields on ubiquitous objects like User is a
common form of in-process caching, so this is a B/C break; restore for now.

PHP allows creating an array in a previously non-existent object property
with $o->foo['bar'] = $val, but doesn't properly handle that on objects
which have magic getter/setter. Add an ugly hack to make it work (but warn).

Depends on I15090ae9e4b66ac25f631f6179c4394ce8c445a9.

Bug: T227688
Change-Id: I62b80ab4fa10de984cf2c879ab12d91b0fd9bc1c