swift234

Make a temporary existential when reflecting weak properties

When getting a mirror child that is a class existential, there
may be witness tables for the protocol composition to copy. Don't
just take the address of a class instance pointer from the stack -
make a temporary existential-like before calling into the Mirror
constructor.

This now correctly covers reflecting weak optional class types, and weak
optional class existential types, along with fixing a stack buffer
overflow reported by the Address Sanitizer (thanks, ASan!).

Tests were also updated to check for the validity of the child's data.

rdar://problem/27348445