Avoid substituting Boolean term variables (#3022)

Avoid substituting Boolean term variables (#3022)

Fixes #3020. Boolean terms that appear in other terms, e.g. a Boolean
array index, are replaced by `BOOLEAN_TERM_VARIABLE`s to make sure that
they are handled properly in theory combination. When doing this
replacement, an equality of the form `(= <Boolean term> <Boolean term
variable)` is added to the assertions. The problem was that
`Theory::ppAssert()` would derive a substitution when this equality was
registered. The commit fixes the problem by not allowing to add
substitutions for `BOOLEAN_TERM_VARIABLE`s.