[mlir][Linalg] NFC - Make markers use Identifier instead of StringRef

Summary: This removes string ownership worries by putting everything into the context and allows more constructing identifiers programmatically.

Reviewers: ftynse

Subscribers: mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, stephenneuendorffer, Joonsoo, grosul1, frgossen, Kayjukh, jurahul

Tags: #mlir

Differential Revision: https://reviews.llvm.org/D81027