[MLIR][Presburger] Add values to PresburgerSpace

This patch allows attaching user information, called "values" to each
identifier. The values are used to carry information along with variables and
are also used to determine if two variables are identical.

This patch is part of a series of patches to allow attaching user information
with variables in Presburger library.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D127347