[analyzer][UninitializedObjectChecker] New flag to turn off dereferencing

Even for a checker being in alpha, some reports about pointees held so little
value to the user that it's safer to disable pointer/reference chasing for now.
It can be enabled with a new flag, in which case checker should function as it
has always been. This can be set with `CheckPointeeInitialization`.

Differential Revision: https://reviews.llvm.org/D49438

llvm-svn: 339135