[ObjC][ARC] Invalidate an entry of UnderlyingObjCPtrCache when the

instruction the key points to is deleted

Use weak value handles for both the key and the value. The entry is
invalid if either value handle is null.

This fixes an assertion failure in BasicAAResult::alias that is caused
by UnderlyingObjCPtrCache returning a wrong value.

I don't have a test case for this patch that fails reliably.

rdar://83984790