[analyzer] Assume that reference symbols are non-null.

By doing this in the constraint managers, we can ensure that ANY reference
whose value we don't know gets the effect, even if it's not a top-level
parameter.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162246 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed