[MLIR][Presburger] IntegerPolyhedron: add support for symbolic integer lexmin

Add support for computing the symbolic integer lexmin of a polyhedron.
This finds, for every assignment to the symbols, the lexicographically
minimum value attained by the dimensions. For example, the symbolic lexmin
of the set

`(x, y)[a, b, c] : (a <= x, b <= x, x <= c)`

can be written as

```
x = a if b <= a, a <= c
x = b if a <  b, b <= c
```

This also finds the set of assignments to the symbols that make the lexmin unbounded.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D122985