Rename SDBMPositiveExpr to SDBMTermExpr

This better reflects how this kind of expressions is used and avoids the
potential confusion since the expression can take negative values.  Term
expressions comprise dimensions, symbols and stripe expressions.  In an SDBM
domain, a stripe expression always corresponds to a variable, input or
temporary.  This expression can appear anywhere an input variable can,
including on the LHS of other stripe expressions.

PiperOrigin-RevId: 268486066