[clang][dataflow] Fix SAT solver crashes on `X ^ X` and `X v X`

BooleanFormula::addClause has an invariant that a clause has no duplicated
literals. When the solver was desugaring a formula into CNF clauses, it
could construct a clause with such duplicated literals in two cases.

Reviewed By: sgatev, ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D130522