[analyzer] Return an UnknownVal when we try to get the binding for a VLA.

This happens in C++ mode right at the declaration of a struct VLA;
MallocChecker sees a bind and tries to get see if it's an escaping bind.
It's likely that our handling of this is still incomplete, but it fixes a
crash on valid without disturbing anything else for now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158587 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed