[analyzer] Set concrete offset bindings to UnknownVal when processing symbolic offset binding, even if no bindings are present.

This addresses an undefined value false positive from concreteOffsetBindingIsInvalidatedBySymbolicOffsetAssignment.

Fixes PR14877; radar://12991168.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177905 91177308-0d34-0410-b5e6-96231b3b80d8