[analyzer] Fix assertion failure after getKnownValue call

Depends on D126560. `getKnownValue` has been changed by the parent patch
in a way that simplification was removed. This is not correct when the
function is called by the Checkers. Thus, a new internal function is
introduced, `getConstValue`, which simply queries the constraint manager.
This `getConstValue` is used internally in the `SimpleSValBuilder` when a
binop is evaluated, this way we avoid the recursion into the `Simplifier`.

Differential Revision: https://reviews.llvm.org/D127285