[mlir][gpu] Refactor functions for workgroup and private buffer attributions.

Summary:

Consolidate interfaces adding workgroup and private buffer attributions in GPU
dialect.

Note all private buffer attributions must follow workgroup buffer attributions.

Reviewers: herhut

Subscribers: mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, csigg, arpith-jacob, mgester, lucyrfox, liufengdb, stephenneuendorffer, Joonsoo, grosul1, frgossen, Kayjukh, llvm-commits

Tags: #llvm, #mlir

Differential Revision: https://reviews.llvm.org/D79508