[analyzer] dynamic_cast Simplify null value generation.

As per Jordy's review. Creating a symbol here is more flexible; however
I could not come up with an example where it was needed. (What
constrains can be added on of the symbol constrained to 0?)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154542 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed