[MLIR][Presburger] Rename attachments to identifiers in PresburgerSpace

"attachment" was a temporary name chosen for the information attached to a
variable in a PresburgerSpace. After the disambiguation of "variables" and
"identifiers" in PresburgerSpace, we use the word "identifiers" for this
information, since this information is used to "identify" these variables.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D128751