[analyzer][solver] Handle simplification to ConcreteInt

The solver's symbol simplification mechanism was not able to handle cases
when a symbol is simplified to a concrete integer. This patch adds the
capability.

E.g., in the attached lit test case, the original symbol is `c + 1` and
it has a `[0, 0]` range associated with it. Then, a new condition `c == 0`
is assumed, so a new range constraint `[0, 0]` comes in for `c` and
simplification kicks in. `c + 1` becomes `0 + 1`, but the associated
range is `[0, 0]`, so now we are able to realize the contradiction.

Differential Revision: https://reviews.llvm.org/D110913