[MLIR][Presburger] Improve unittest parsing

This patch adds better functions for parsing MultiAffineFunctions and
PWMAFunctions in Presburger unittests.

A PWMAFunction can now be parsed as:

```
PWMAFunction result = parsePWMAF({
    {"(x, y) : (x >= 10, x <= 20, y >= 1)", "(x, y) -> (x + y)"},
    {"(x, y) : (x >= 21)", "(x, y) -> (x + y)"},
    {"(x, y) : (x <= 9)", "(x, y) -> (x - y)"},
    {"(x, y) : (x >= 10, x <= 20, y <= 0)", "(x, y) -> (x - y)"},
});
```

which is much more readable than the old format since the output can be
described as an AffineMap, instead of coefficients.

This patch also adds support for parsing divisions in MultiAffineFunctions
and PWMAFunctions which was previously not possible.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D133654