[analyzer] Add a simple check for initializing reference variables with null.

[analyzer] Add a simple check for initializing reference variables with null.

There's still more work to be done here; this doesn't catch reference
parameters or return values. But it's a step in the right direction.

Part of <rdar://problem/11212286>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161214 91177308-0d34-0410-b5e6-96231b3b80d8