[mlir] FlatAffineConstraint parsing for unit tests

This patch adds functionality to parse FlatAffineConstraints from a
StringRef with the intention to be used for unit tests. This should
make the construction of FlatAffineConstraints easier for testing
purposes.

The patch contains an example usage of the functionality in a unit test that
uses FlatAffineConstraints.

Reviewed By: bondhugula, grosser

Differential Revision: https://reviews.llvm.org/D113275