[mlir] Avoid pontentially ambiguous class name

Summary: The Pass class exists in both the mlir and the llvm namespaces. Use the fully qualified class name to avoid any ambiguities.

Reviewers: rriddle

Reviewed By: rriddle

Subscribers: mehdi_amini, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, stephenneuendorffer, Joonsoo, grosul1, jurahul, msifontes

Tags: #mlir

Differential Revision: https://reviews.llvm.org/D82371