Overload arithmetic operators for SDBM expressions

    Provide an "unsafe" version of the overloaded arithmetic operators for SDBM
    expressions.  These operators expect the operands to be of the right SDBM
    expression subtype and assert if they are not.  They also perform simple
    folding operations as well as some semantically correct operations that
    construct an SDBM expression of a different subtype, e.g., a difference
    expression if the RHS of an operator+ is a negated variable.  These operators
    are scoped in a namespace to allow for a future "safe" version of the operators
    that propagates null expressions to denote the error state when expressions
    have wrong subtypes.

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PiperOrigin-RevId: 248704153