[MLIR][Presburger][NFC] Use "disjunct" to refer to disjuncts in PresburgerRelation

This patch modifies the name "integerRelations" and "relation" to refer to the
disjuncts in PresburgerRelation to "disjunct(s)".  This is done to be
consistent with the rest of the interface.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D122892