[Strings] Make fact detection more robust (#7007)

Currently, our check for whether an inference is a fact or a lemma
involves checking whether the kind of the conclusion is a conjunction or
a disjunction. However, this does not take into account inferences of
other kinds such as ites, which is a problem because they require a
decision from the SAT solver. This commit changes the condition to check
the theory of the conclusion. If the conclusion belongs to the theory of
strings, it considers it as a fact.

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