[MLIR][Presburger] Implement function to evaluate the number of terms in a...

[MLIR][Presburger] Implement function to evaluate the number of terms in a generating function. (#78078)

We implement `computeNumTerms()`, which counts the number of terms in a
generating function by substituting the unit vector in it.
This is the main function in Barvinok's algorithm – the number of points
in a polytope is given by the number of terms in the generating function
corresponding to it.
We also modify the GeneratingFunction class to have `const` getters and
improve the simplification of QuasiPolynomials.