Create a gpu.module operation for the GPU Dialect.

Summary:
This is based on the use of code constantly checking for an attribute on
a model and instead represents the distinct operaion with a different
op. Instead, this op can be used to provide better filtering.

Reverts "Revert "[mlir] Create a gpu.module operation for the GPU Dialect.""

This reverts commit ac446302 after
fixing internal Google issues.

This additionally updates ROCDL lowering to use the new gpu.module.

Reviewers: herhut, mravishankar, antiagainst, nicolasvasilache

Subscribers: jholewinski, mgorny, mehdi_amini, jpienaar, burmako, shauheen, csigg, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, llvm-commits, mravishankar, rriddle, antiagainst, bkramer

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72921