[mlir][Linalg] NFC - Rename ViewTraits -> StructuredOpTraits

Summary: This is part of an ongoing cleanup and uniformization work.

Reviewers: ftynse

Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72079